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S Chand Class 11 ICSE Maths Solutions Chapter 24 Ellipse Ex 24

Question 1.
Find the eccentricity of the ellipse of which the major axis is double the minor axis.
Solution:
Let a be the length of semi-major and semi-minor axis of the ellipse.
According to given condition, a = 2b
We know that b² = a² (1 – e²)
⇒ b² = 4b²(1 – e²)
⇒ \(\frac { 1 }{ 4 }\) = 1 – e² ⇒ e² = 1 – \(\frac { 1 }{ 4 }\) = \(\frac { 3 }{ 4 }\)
⇒ e = \(\frac{\sqrt{3}}{2}\) [∵ e > 0]
Thus required eccentricity of an ellipse be \(\frac{\sqrt{3}}{2}\).

Question 2.
If the minor axis of an ellipse is equal to the distance between its foci, prove that its eccentricity is \(\frac{1}{\sqrt{2}}\).
Solution:
Let e be the eccentricity of an ellipse according to given condition, we have
2b = 2ae ⇒ b = ae
We know that b² = a² (1 – e²)
⇒ a²e² = a² (1 – e²)
⇒ e² = 1 – e²
⇒ 2e² = 1 ⇒ e = \(\frac{1}{\sqrt{2}}\) [∵ e > 0]

Question 3.
Find the latus rectum and eccentricity of the ellipse whose semi-axes are 5 and 4.
Solution:
Given a = 5 and b = 4
Let e be the eccentricity of an ellipse Then b² – a² (1 – e²)
⇒ 16 = 25 (1 – e²)
⇒ 1 – e² = \(\frac { 16 }{ 25 }\) ⇒ e² = \(\frac { 9 }{ 25 }\)
⇒ e = \(\frac { 3 }{ 5 }\) (∵ e > 0)
∴ length of latus-rectum = \(\frac{2 b^2}{a}\)
= 2 × \(\frac{4^2}{5}\) = \(\frac { 32 }{ 5 }\)

Question 4.
Find the eccentricity of the ellipse whose latus rectum is (/) half its major axis, (ii) half its minor axis.
Solution:
Let a be the length of semi-major and b be the length of semi-minor axis of an ellipse and e be the eccentricity of an ellipse.
(i) According to given condition,

Question 5.
If the eccentricity is zero, prove that the ellipse becomes a circle.
Solution:
We know that, b² = a² (1 – e²) …(1)
We have given e = 0 ∴ from (1) ; we have
b² = a² b = a (∵ b, a > 0)
Thus the given eqn. of ellipse reduces to,
\(\frac{x^2}{a^2}\) + \(\frac{y^2}{a^2}\) = 1
⇒ x² + y² = a²

Question 6.
Find the equation to the ellipse with axes as the axes of coordinates.
(i) major axis = – 6, minor axis = 4 ;
(ii) which passes through the points (- 3, 1) and (2, – 2) ;
(iii) axes are 10 and 8 and the major axis along
(a) the axis of x,
(b) the axis of y ;
(iv) major axis \(\frac{9}{2}\) and eccentricity \(\frac{1}{\sqrt{3}}\), where the major axis is the horizontal axis ;
(v) latus rectum is 5 and eccentricity \(\frac{2}{3}\),
(vi) foci are (± 4, 0) and e = \(\frac{1}{3}\);
(vii) distance between the foci is 10 and its latus rectum is 15 ;
(viii) distance of the focus from the corresponding directrix is 9 and eccentricity is \(\frac{4}{5}\);
(ix) the minor axis is equal to the distance between the foci, and the latus rectum is 10.
Solution:
(i) Let a and b are the lengths of major and minor axes of an ellipse respectively and the eqn. of ellipse be
\(\frac{x^2}{a^2}\) + \(\frac{y^2}{b^2}\) = 1 …(1)
where a > b > 0
given 2a = 6 ⇒ a = 3
and 2b = 4 ⇒ b = 2
∴ eqn. (1) reduces to;
\(\frac{x^2}{9}\) + \(\frac{y^2}{4}\) = 1
⇒ 4x² + 9y² = 36

(ii) Let the eqn. of ellipse be
\(\frac{x^2}{a^2}\) + \(\frac{y^2}{b^2}\) = 1, where a > b > 0 …(1)
eqn. (1) pass through the points (- 3, 1) and (2, – 2)
∴ \(\frac{9}{a^2}\) + \(\frac{1}{b^2}\) = 1 …(2)
\(\frac{4}{a^2}\) + \(\frac{4}{b^2}\) = 1 …(2)
4 × eqn. (2) – eqn. (3); we have
\(\frac{32}{a^2}\) = 3 ⇒ a² = \(\frac{32}{3}\)
∴ from (2); \(\frac{9 \times 3}{32}\) + \(\frac{1}{b^2}\) = 1
⇒ \(\frac{1}{b^2}\) = \(\frac{5}{32}\)
⇒ b² = \(\frac{32}{5}\)
Thus eqn. (2) reduces to;
\(\frac{3 x^2}{32}\) + \(\frac{5 y^2}{32}\) = 1;
⇒ 3x² + 5y² = 32

(iii) (a) Here, major axis along x-axis and minor axis along y-axis. Let the eqn. of ellipse be,
\(\frac{x^2}{a^2}\) + \(\frac{y^2}{b^2}\) = 1, where a > b > 0 …(1)
given 2a = 10 ⇒a = 5
and 2b = 8 ⇒ b = 4
∴ eqn. (1) reduces to;
\(\frac{x^2}{25}\) + \(\frac{y^2}{16}\) = 1 ⇒ 16x² + 25y² = 400

(b) Here, major axes along y-axis and minor axis along x-axis.
Let the eqn. of ellipse be taken as
\(\frac{x^2}{b^2}\) + \(\frac{y^2}{a^2}\) = 1 …(1)
when a> b> 0
according to given a = 5 and b = 4
∴eqn. (1) reduces to ; \(\frac{x^2}{16}\) + \(\frac{y^2}{25}\) = 1
⇒ 25x² + 16y² = 400
which is the required eqn. of an ellipse,

(iv) Since the major axis is the horizontal axis and let the eqn. of ellipse can be taken as
\(\frac{x^2}{a^2}\) + \(\frac{y^2}{b^2}\) = 1 …(1)
wher a > b > 0
given 2a = \(\frac{9}{2}\) ⇒ a = \(\frac{9}{4}\) and e = \(\frac{1}{\sqrt{3}}\)
We know that b² = a² (1 – e² )
⇒ b² = \(\left(\frac{9}{4}\right)^2\left[1-\frac{1}{3}\right]\) = \(\frac{81}{16}\) × \(\frac{2}{3}\) = \(\frac{27}{8}\)
Thus eqn. (1) redues to ;
\(\frac{16 x^2}{81}\) + \(\frac{8 y^2}{27}\) = 1 ⇒ 16x² + 24y² = 81
which is the required eqn. of an ellipse,

(v) Let a and b are the lengths of semi-major and semi-minor axes of an ellipse. Let e be the eccentricity of an ellipse. Let the eqn. of ellipse be,
\(\frac{x^2}{a^2}\) + \(\frac{y^2}{b^2}\) = 1 …(1)
given e = \(\frac{2}{3}\) and \(\frac{2 b^2}{a}\) = 5
⇒ b² = \(\frac{5 a}{2}\)
We know that, b² = a²(1 – e²)
⇒ \(\frac{5 a}{2}\) = a² \(\left(1-\frac{4}{9}\right)\) = \(\frac{5 a^2}{9}\)
⇒ a = \(\frac{9}{2}\)
∴ from (1) ; b² = \(\frac{5}{2}\) × \(\frac{9}{2}\) = \(\frac{45}{4}\)
Putting the value of a and b in eqn. (1); we have
\(\frac{4 x^2}{81}\) + \(\frac{4 y^2}{45}\) = 1
⇒ \(\frac{20 x^2+36 y^2}{405}\) = 1
⇒ 20x² + 36y² = 405
Which is the required eqn. of an ellipse.

(vi) given foci are (± 4, 0) i.e. both foci lies on x-axis and hence x-axis be the major axes.
Thus eqn. of ellipse can be written as \(\frac{x^2}{a^2}\) + \(\frac{y^2}{b^2}\) = 1 …(1)
where a > b > 0
Its foci are (± ae, 0) ∴ ae = 4
and also given e = \(\frac{1}{3}\)
∴ a = 12
We know that b² = a² (1 – e²)
= 144 \(\left(1-\frac{1}{9}\right)\) = 144 × \(\frac{8}{9}\)
⇒ b² = 128
Thus, eqn.(1) reduces to;
\(\frac{x^2}{144}\) + \(\frac{y^2}{128}\) = 1 ⇒ 8x² + 9y² – 1152 = 0
which is the required eqn. of an ellipse.

(vii) Let a and b be the length of semi-major and semi-minor axes of an ellipse and let the eqn. of ellipse can be taken as
\(\frac{x^2}{a^2}\) + \(\frac{y^2}{b^2}\) = 1 …(1)
where a > b > 0
given distance between foci = 10
⇒ 2ae = 10 ⇒ ae = 5 …(2)
and \(\frac{2 b^2}{a}\) = 15 ⇒ b² = \(\frac{15a}{2}\) …(3)
We know that b² = a² (1 – e²)
⇒ \(\frac{15a}{2}\) = a² – 25
⇒ 2a² – 15a – 50 = 0
⇒ 2a² – 20a + 5a – 50 = 0
⇒ 2a (a – 10) + 5 (a – 10) = 0
⇒ (a-10) (2a + 5) = 0
⇒ a= 10 (∵ a > 0)
∴ from (3); b² = \(\frac{15 \times 10}{2}\) = 75
Thus eqn. (1) reduces to ;
\(\frac{x^2}{100}\) + \(\frac{y^2}{75}\) = 1
⇒ 3x² + 4y² = 300
which is the required eqn. of an ellipse.

(viii) Given distance of focus (ae, 0) from
directrix x – \(\frac{a}{e}\) = 0 = 9 (given)

(ix) Let a be the length of semi-major axis and b be the length of semi-minor axis and let e be the eccentricity of required ellipse.
Let the required eqn. of an ellipse be,
\(\frac{x^2}{a^2}\) + \(\frac{y^2}{b^2}\) = 1 …(1)
where a > b > 0
According to given condition,
2b = 2 ae ⇒ b = ae …(2)
and \(\frac{2 b^2}{a}\) = 10
⇒ b² = 5a …(3)
We know that, b² = a² (1 – e²)
⇒ b² = a² – b²
⇒ 2b² = a²
⇒ 2 × 5a = a² [using (2) and (3)]
⇒ a = 10
∴ b² = 5 × 10 = 50
Thus eqn. (1) reduces to ;
\(\frac{x^2}{100}\) + \(\frac{y^2}{50}\) = 1
⇒ x² + 2y² = 100
which is the required eqn. of an ellipse.

Question 7.
Find the equation of the ellipse whose centre is at (- 2, 3) and whose semi-axes are 3 and 2, when the major axis is
(i) parallel to the axes of x ;
(ii) parallel to the axis of y.
Solution:
(i) Since the centre of required ellipse be C (- 2, 3) and mojor axis is parallel to x-axis.
Hence the eqn. of ellipse can be taken as,
\(\frac{(x+2)^2}{9}\) + \(\frac{(y-3)^2}{b^2}\) = 1 …(1)
where a > b > 0
Clearly length of semi-major axis = a = 3
and length of semi-minor axis = b = 2
∴ eqn. (1) reduces to,
\(\frac{(x+2)^2}{9}\) + \(\frac{(y-3)^2}{4}\) = 1
⇒ 4 (x² + 4x + 4) + 9 (y² – 6y + 9) = 36
⇒ 4x² + 9y² + 16x – 54y + 61 = 0
which is the required eqn. of ellipse.

(ii) The eqn. of ellipse having centre C (-2, 3) and major axis is parallel to y-axis be taken as
\(\frac{(x+2)^2}{b^2}\) + \(\frac{(y-3)^2}{a^2}\) = 1 …(1)
where a > b > 0
given length of semi-major axis = a = 3
and length of semi-minor axis = b = 2
∴ eqn. (1) reduces to,
\(\frac{(x+2)^2}{4}\) + \(\frac{(y-3)^2}{9}\) = 1
⇒ 9 (x + 2)² + 4(y – 3)² = 36
⇒ 9x² + 4y² + 36x – 24y + 36 = 0
which is the required eqn. of an ellipse.

Question 8.
Find the equation of the ellipse with its centre at (4, – 1), focus at (1, – 1), and passing through (8, 0).
Solution:
Let S’ (α, β) be the other foci of the required ellipse. Thus C (4, -1) be the mid-point of SS’.

Thus the coordinates of other foci are S’ (7, – 1).
Since the ordinate of points S and S’ are equal. Thus major axis is parallel to x-axis and it is a horizontal ellipse.
Let the eqn. of ellipse be
\(\frac{(x-4)^2}{a^2}\) + \(\frac{(y+1)^2}{b^2}\) = 1 …(1)
where a > b > 0
Distance between foci = 2ae = 6 ⇒ ae = 3
We know that b² = a² (1 – e²) = a² – 9
Thus eqn. (1) reduces to;
\(\frac{(x-4)^2}{a^2}\) + \(\frac{(y+1)^2}{a^2-9}\) = 1 …(2)
eqn. (2) passes through the point (8, 0), we get
\(\frac{(8-4)^2}{a^2}\) + \(\frac{1}{a^2-9}\) = 1
⇒ \(\frac{16}{a^2}\) + \(\frac{1}{a^2-9}\) = 1
⇒ 16 (a² – 9) + a² = a² (a² – 9)
⇒ 17a² – 144 = a⁴ – 9a²
⇒ a⁴ – 26a² + 144 = 0
⇒ a⁴ – 18a² – 8a² + 144 = 0
⇒ a² (a² – 18)- 8 (a² – 18) = 0
⇒ (a² – 18) (a² – 8) = 0
⇒ a² =18, 8
When a² = 8 ∴ b² = a² – 9 = 8 – 9 = – 1
which is false
Thus a² = 18 ∴ eqn. (2) reduces to ;
\(\frac{(x-4)^2}{18}\) + \(\frac{(y+1)^2}{18-9}\) = 1
⇒ \(\frac{(x-4)^2}{18}\) + \(\frac{(y+1)^2}{9}\) = 1
⇒ (x – 4)² + 2 (y + 1)² = 18
⇒ x² + 2y² – 8x +4y = 0
which is the required eqn. of ellipse.

Question 9.
Find the equation of the ellipse with its centre at (3, 1), vertex at (3, – 2), and eccentricity equal to \(\frac{1}{3}\) .
Solution:
Since the centre of required ellipse be C (3, 1) and vertex A (3, – 2). Let the other vertex of ellipse be A’ (α, β). Then C (3, 1) be the mid point of line segment A A’.
∴ 3 = \(\frac{\alpha+3}{2}\) ⇒ α = 3
and 1 = \(\frac{\beta-2}{2}\) ⇒ β = 3
∴ Coordinates of other vertex of an ellipse be A’ (3, 4).
Also x-coordinates of A and A’ are equal
∴ major axis of required ellipse is parallel to y-axis.
Now a = | CA | = | CA’ | = 3
and e = eccentricity of ellipse = \(\frac{1}{3}\)
∴ b² = a² (1 – e)² = 9 \(\left(1-\frac{1}{9}\right)\) = \(\frac{9 \times 8}{9}\) = 8
Thus required eqn. of ellipse having centre C (3, 1) and major axis parallel toy-axis is given by
\(\frac{(x-3)^2}{b^2}\) + \(\frac{(y-1)^2}{a^2}\) = 1
⇒ \(\frac{(x-3)^2}{8}\) + \(\frac{(y-1)^2}{9}\) = 1
⇒ 9 (x – 3)²+ 8(y – 1)² = 72
⇒ 9x² + 8y² – 54x – 16y + 17 = 0

Question 10.
Find the equation of the ellipse whose centre is at (0, 2) and major axis along the axis of y and whose minor axis is equal to the distance between the foci and whose latus rectum is 2.
Solution:
Let the eqn. of ellipse with centre at (0, 2) and having major axis along y-axis be given by
\(\frac{(x-0)^2}{b^2}\) + \(\frac{(y-2)^2}{a^2}\) = 1 …(1)
where a > b > 0
Also, 2b = 2ae ⇒ b = ae …(2)
Further \(\frac{2 b^2}{a}\) = 2 ⇒ b² = a …(3)
We know that, b² = a² (1 – e²)
⇒ a = a² – a (using (2) and (3)]
⇒ 2a = a² ⇒ a = 2 [∵ a > 0]
from (3); b² = 2
Thus eqn. (1) reduces to ;
\(\frac{x^2}{2}\) + \(\frac{(y-2)^2}{4}\) = 1
⇒ 2x² + (y – 2)² = 4
⇒ 2x² + y² – 4y = 0
which is the required eqn. of an ellipse.

Question 11.
Find the equation of the ellipse with (i) focus at (1, – 1), directrix x = 0, and e = \(\frac{\sqrt{2}}{2}\);
(ii) focus at (0, 0), eccentricity is \(\frac{5}{6}\), and directrix is 3x + 4y – 1 = 0.
Solution:
(i) Let P (x, y) be any point on the parabola s.t | PF | = e | PM |
\(\sqrt{(x-1)^2+(y+1)^2}\) = \(\frac{\sqrt{2}|x|}{2}\)
On squaring both sides ; we have
(x – 1)² + (y + 1)² = \(\frac { 1 }{ 2 }\)x²
⇒ 2 [(x – 1)² + (y + 1)²] = x²
⇒ x² + 2y² – 4x + 4y + 4 = 0
which is the required eqn. of an ellipse.

(ii) Let P (x, y) be any point on ellipse
Then by def. | PF | = e | PM |

On squaring both sides ; we have
36(x² + y²) = (3x + 4y – 1)²
⇒ 36 x² + 36 y² = 9x² + 16y² + 1 + 24xy – 8y – 6x
⇒ 27x² + 20y2 – 24xy + 8y + 6x – 1 = 0
which is the required eqn. of an ellipse.

Question 12.
Find the equation of the ellipse from the following data : axis is coincident with x = 1, centre is (1, 5), focus is (1, 8) and the sum of the focal distances of a point on the ellipse is 12.
Solution:
Let the eqn. of ellipse be
\(\frac{(x-1)^2}{b^2}\) + \(\frac{(y-5)^2}{a^2}\) = 1 …(1)
Since axis of ellipse is coincident with x = 1 i.e. required ellipse be a vertical ellipse, given centre of ellipse be C (1, 5) and Focus be S (1, 8).
∴ | CS | = \(\sqrt{(1-1)^2+(8-5)^2}\) = 3
⇒ ae = 3 …(2)
Further sum of focal distances from any point on the ellipse = |PS| + |PS’| = 2a = 12
⇒ a = 6
∴ from (2); e = \(\frac { 3 }{ 6 }\) = \(\frac { 1 }{ 2 }\)
We know that b² = a² (1 – e²)
⇒ b² = 36\(\left(1-\frac{1}{4}\right)\)
⇒ b² = 36 × \(\frac { 3 }{ 4 }\) = 27
∴ eqn. (1) reduces to ;
\(\frac{(x-1)^2}{27}\) + \(\frac{(y-5)^2}{36}\) = 1
which is the required eqn. of an ellipse.

Question 13.
A point P (x, y) moves so that the product of the slopes of the two lines joining P to the two points (- 2, 1) and (6, 5) is – 4. Show that the locus is an ellipse and locate its centre.
Solution:
Let the given points are A (- 2, 1) and B (6, 5).
slope of line joining P (x, y) and A (- 2, 1) = \(\frac{y-1}{x+2}\) = m₁
slope of line joining P (x, y) and B (6, 5) = \(\frac{y-5}{x-6}\) = m₂
product of slopes of both lines = – 4
⇒ \(\left(\frac{y-1}{x+2}\right)\) \(\left(\frac{y-5}{x-6}\right)\) = – 4
⇒ (y – 1) (y – 5) + 4 (x + 2) (x – 6) = 0
⇒ y² + 4x² – 6y – 16y – 43 = 0
which is the required eqn. of an ellipse.
4x² – 16x + y² – 6y = 43
⇒ 4 (x² – 4x + 4 – 4) + (y² – 6y + 9 – 9) = 43
⇒ 4 (x – 2)² + (y – 3)² = 68
⇒ \(\frac{(x-2)^2}{17}\) + \(\frac{(y-3)^2}{68}\) = 1
Clearly centre of an ellipse be C (2, 3).

Question 14.
Find the eccentricity, the coordinates of the foci, and the length of the latus rectum of the ellipse 2x² + 3y² = 1.
Solution:
Given eqn. of an ellipse be
2x² + 3y² = 1
⇒ \(\frac{x^2}{1 / 2}\) + \(\frac{y^2}{1 / 3}\) = 1 …(1)
On comparing with \(\frac{x^2}{a^2}\) + \(\frac{y^2}{b^2}\) = 1,
where a > b > 0
a² = \(\frac{1}{2}\); b² = \(\frac{1}{3}\)
We know that b² = a² (1 – e²)
⇒ \(\frac{1}{3}\) = \(\frac{1}{2}\) (1 – e²) ⇒ \(\frac{2}{3}\) = 1 – e²
⇒ e² = 1 – \(\frac{2}{3}\) = \(\frac{1}{3}\)
⇒ e = \(\frac{1}{\sqrt{3}}\) [∵ e > 0]
Thus required eccentricity of an ellipse be \(\frac{1}{\sqrt{3}}\). Foci are given by (± ae, 0)

Question 15.
For the ellipse, 9x² + 16y² = 576, find the semi-major axis, the semi-minor axis, the eccentricity, the coordinates of the foci, the equations of the directrices, and the length of the latus rectum.
Solution:
Given eqn. of an ellipse be
9x² + 16y² = 576
⇒ \(\frac{x^2}{64}\) + \(\frac{x^2}{64}\) = 1 ….(1)
On comparing with \(\frac{x^2}{a^2}\) + \(\frac{y^2}{b^2}\) = 1;
where a > b > 0
∴ a² = 64 and b² = 36
We know that b² = a² (1 – e²)
⇒ 36 = 64 (1 – e²)
⇒ \(\frac{36}{64}\) = 1 – e²
⇒ e² = 1 – \(\frac{36}{64}\) = 1 – \(\frac{9}{16}\) = \(\frac{7}{16}\)
∴ e = \(\frac{\sqrt{7}}{4}\) [∵ e > 0]
Thus required eccentricity of an ellipse be \(\frac{\sqrt{7}}{4}\)
Here, a = 8 ; b = 6 (∵ a > b > 0)
∴ length of semi major-axis = a = 8
and length of semi minor axis = b = 6
foci are given by (± ae, 0)
i.e. \(\left( \pm 8 \times \frac{\sqrt{7}}{4}, 0\right)\) i.e. \(( \pm 2 \sqrt{7}, 0)\)
length of latus rectum = \(\frac{2 b^2}{a}\) = \(\frac{2 \times 36}{8}\) = 9
eqns. of directrices are given by x = ±\(\frac{a}{e}\)
i.e. x = ±\(\frac{8}{\sqrt{7}}\) × 4 = ±\(\frac{32}{\sqrt{7}}\).

Question 16.
Find the length of the axes, the co-ordinates of the foci, the eccentricity, and latus rectum of the ellipse 3x² + 2y² = 24.
Solution:
Given eqn. of ellipse be 3x² + 2y² = 24
⇒ \(\frac{x^2}{8}\) + \(\frac{y^2}{12}\) = 1;
where a > b > 0
On comparing with \(\frac{x^2}{b^2}\) + \(\frac{y^2}{a^2}\) = 1
we have a² = 12 ; b² = 8
i.e. a = 2√3
and b = 2√2
length of major axis = 2a = 4√3
and length of minor axis = 2b = 4√2
we know that b2 = a² (1 – e²)
⇒ 8 = 12(1 – e²)
⇒ 1 – e² = \(\frac{2}{3}\)
⇒ e² = \(\frac{1}{3}\)
⇒ e = \(\frac{1}{\sqrt{3}}\) (∵e > 0)
Thus foci of an ellipse be (0, ± ae)
i.e. \(\left(0, \pm 2 \sqrt{3} \times \frac{1}{\sqrt{3}}\right)\) i.e. (0, ± 2)
length of latus-rectum = \(\frac{2 b^2}{a}\) = \(\frac{2 \times(2 \sqrt{2})^2}{2 \sqrt{3}}\) = \(\frac{8}{\sqrt{3}}\)

Question 17.
Find the eccentricity of the ellipse, 4x² + 9y² – 8x – 36y + 4 = 0.
Solution:
Given eqn. of an ellipse be,
4x² + 9y² – 8x – 36y + 4 = 0
⇒ 4(x² – 2x + 1 – 1) + 9(y² – 4y + 4 – 4) + 4 = 0
⇒ 4 [(x – 1)² – 1] + 9 [y – 2)² – 4] + 4 = 0
⇒ 4 (x – 1)² + 9 (y – 2)² = 36
⇒ \(\frac{(x-1)^2}{9}\) + \(\frac{(y-2)^2}{4}\) = 1 …(1)

On comparing with \(\frac{x^2}{a^2}\) + \(\frac{y^2}{b^2}\) = 1 ; where a > b > 0
Here, a² = 9 and b² = 4
we know that, b² = a² (1 – e²) ⇒ 4 = 9 (1 – e²) ⇒ 1 – e² = \(\frac{4}{9}\)
⇒ e² = \(\frac{5}{9}\) ⇒ e = \(\frac{\sqrt{5}}{3}\) (∵ e > 0)
Thus, required eccentricity of an ellipse be \(\frac{\sqrt{5}}{3}\).

Question 18.
find the centre of the ellipse, \(\frac{x^2-a x}{a^2}\) + \(\frac{y^2-b y}{b^2}\) = 0.
Solution:
Given eqn. of an ellipse be

Question 19.
Find the distance between a focus and an extremity of the minor axis of the ellipse
(i) 4x² + 5y² = 100
(ii) \(\frac{x^2}{a^2}\) + \(\frac{y^2}{b^2}\) = 1
Solution:
(i) Given eqn. of ellipse be 4x² + 5y² = 100
⇒ \(\frac{x^2}{25}\) + \(\frac{y^2}{20}\) = 1 …(1)
On comparing eqn. (1) with \(\frac{x^2}{a^2}\) + \(\frac{y^2}{b^2}\) = 1
we have
where a > b > 0
a² = 25 and b² = 20
We know that b² = a² (1 – e²)
⇒ 20 = 25 (1 – e²)
⇒ \(\frac{4}{5}\) = 1 – e²
⇒ e² = 1 – \(\frac{4}{5}\) = \(\frac{1}{5}\)
⇒ e = \(\frac{1}{\sqrt{5}}\) (∵ e > 0)
∴ required distance = distance between focus (ae, 0) and (0, b)

(ii) Given eqn. of ellipse be \(\frac{x^2}{a^2}\) + \(\frac{y^2}{b^2}\) = 1
∴ required distance = \(\sqrt{b^2+a^2 e^2}\) = \(\sqrt{a^2}\) = a
[∵ b² = a² (1 – e)² ⇒ b² + a²e² = a²]

Question 20.
Given the ellipse 36x² + 100y² = 3600, find the equation and the lengths of the focal radii drawn through the point \(\left(8, \frac{18}{5}\right)\).
Solution:
Given eqn. of ellipse be,
\(\frac{x^2}{100}\) + \(\frac{y^2}{36}\) = 1 …(1)
On comparing with \(\frac{x^2}{a^2}\) + \(\frac{y^2}{b^2}\) = 1
we have a² = 100; b² = 36
We know that, b² = a² (1 – e²)
⇒ 36 = 100 (1 – e²) ⇒ 1 – e² = \(\frac{36}{100}\) = \(\frac{9}{25}\)
⇒ e² = 1 – \(\frac{9}{25}\) = \(\frac{16}{25}\)
⇒ e = \(\frac{4}{5}\) (∵ e > 0)
required lengths of focal radii drawn through the point \(\left(8, \frac{18}{5}\right)\) = a ± ex₁
= 10 ± \(\frac{4}{5}\) × 8 = 10 ± \(\frac{32}{5}\) = \(\frac{82}{5}\), \(\frac{18}{5}\)
foci are given by (± ae, 0) i.e. \(\left( \pm 10 \times \frac{4}{5}, 0\right)\) i.e. (± 8, 0)
The eqns. of focal radii joining the points (± 8, 0) and \(\left(8, \frac{18}{5}\right)\) be given by

Question 21.
The focal distance of an end of the minor axis of the ellipse is k and the distance between the foci is 2h. Find the lengths of the semi-axes.
Solution:
Given, the focal distance of an end of minor axis be k.
The ends of minor axes be (0, ± b).
i.e. given a – ex₁ = k ⇒ a – e × 0 = k
⇒ a = k
distance between foci = 2h ⇒ 2ae = 2h
⇒ ae – h ⇒ e = \(\frac{h}{k}\)
We know that b² = a² (1 – e²)
⇒ b² = k² \(\left[1-\frac{h^2}{k^2}\right]\)
⇒ b² = k² – h²
⇒ b = \(\sqrt{k^2-h^2}\)

Question 22.
Find the eccentricity of the ellipse whose latus rectum is 4 and distance of the vertex from the nearest focus is 1.5 cm.
Solution:
Let e be the eccentricity of an ellipse
According to given condition, \(\frac{2 b^2}{a}\) = 4 ⇒ b² = 2a …(1)
Also, a – ae = 1.5 …(2)
Also we know that b² = a² (1 – e²) ⇒ 2a = a² (1 – e²) …(2)
⇒ 2a = a² – \(\left(a-\frac{3}{2}\right)^2\)
⇒ 2a = a² – a² – \(\frac{9}{4}\) + 3a ⇒ a = \(\frac{9}{4}\)
∴ from (2); \(\frac{9}{4}\) (1 – e) = \(\frac{3}{2}\)
⇒ 1 – e = \(\frac{3}{2}\) × \(\frac{4}{9}\)
⇒ 1 – e = \(\frac{2}{3}\) ⇒ e = \(\frac{1}{3}\)
Thus the required eccentricity of an ellipse be \(\frac{1}{3}\).

Question 23.
The directrix of a conic section is the line 3x + 4y = 1 and the focus S is (- 2, 3). If the eccentricity e is \(\frac{1}{\sqrt{2}}\), find the equation to the conic section.
Solution:
Given focus of ellipse be F (- 2, 3) and eqn. of corresponding directrix is 3x + 4y – 1 = 0 and eccentricity e = \(\frac{1}{\sqrt{2}}\).
Let P (x, y) be any point on ellipse and PM be the ⊥ drawn from P on given directrix. Then by definition of ellipse, we have | PF | = e | PM |
⇒ \(\sqrt{(x+2)^2+(y-3)^2}\) = \(\frac{1}{\sqrt{2}}\) \(\frac{|3 x+4 y-1|}{\sqrt{3^2+4^2}}\)
On squaring both sides ; we have
(x + 2)² + (y – 3)² = \(\frac{1}{2}\) × \(\frac{1}{25}\)(3x + 4y – 1)²
⇒ 50 [(x + 2)² + (y – 3)²] = (3x + 4y – 1)²
⇒ 50 (x² + y² + 4x – 6y + 13) = 9x² + 16y² + 1 + 24xy – 8y – 6x
⇒ 41x² + 34x² – 24xy + 206x – 292y + 649 = 0 which is the required eqn. of ellipse.

Question 24.
Find the equation to the conic section whose focus is (1, – 1), eccentricity is \(\left(\frac{1}{2}\right)\) and the directrix is the line x – y = 3. Is the conic section an ellipse ?
Solution:
Given focus of ellipse be F (1, – 1) and eqn. of directrix be x – y – 3 = 0 and e = \(\frac{1}{2}\)
Let P (x, y) be any point on ellipse. Then by def. of ellipse, we have | PF | = e | PM | where PM be the ⊥ drawn from P on given directrix.
⇒ \(\sqrt{(x-1)^2+(y+1)^2}\) = \(\frac{1}{2}\) \(\frac{|x-y-3|}{\sqrt{1^2+(-1)^2}}\)
On squaring both sides, we have
8 [(x – 1)² + (y + 1)²] = (x – y – 3)²
⇒ 8(x² – 2x + y² + 2y + 2) = x² + y² + 9 – 2xy + 6y – 6x
⇒ 7x² + 7y² + 2xy – 10x + 10y + 7 = 0 which is the required eqn. of ellipse.

Question 25.
Find the equation of the ellipse whose foci are (- 1, 5) and (5, 5) and whose major axis is 0.
Solution:
Given foci of an ellipse are S (- 1, 5) and S’ (5, 5).
Since ordinates of both foci are same.
Thus, eqn. of axes of an ellipse be parallel to x-axis and let C (α, β) be the coordinates of centre of an ellipse i.e. C (α, β) be the mid-point of SS’.
Thus, α = \(\frac{-1+5}{2}\) and β = \(\frac{5+5}{2}\)
i.e. a = 2 and p = 5
Let the required eqn. of ellipse be
\(\frac{(x-2)^2}{a^2}\) + \(\frac{(y-5)^2}{b^2}\) = 1 …(1)
given 2a = 10 ⇒ a = 5
Also, 2ae = 6 ⇒ ae = 3 ⇒ e = \(\frac{3}{5}\) < 1
We know that b² = a² (1 – e²)
⇒ b² = 25\(\left(1-\frac{9}{25}\right)\) = 16
Thus eqn. (1) reduces to ;
\(\frac{(x-2)^2}{25}\) + \(\frac{(y-5)^2}{16}\) = 1
which is the required eqn. of an ellipse.

Question 26.
Find the ellipse if its foci are (± 2, 0) and the length of the latus rectum is \(\frac{10}{3}\).
Solution:
Given foci of ellipse are (± 2, 0) and both foci lies on x-axis. So x-axis being the major axes of an ellipse.
Let the eqn. of ellipse be taken as
\(\frac{(x-0)^2}{a^2}\) + \(\frac{(y-0)^2}{b^2}\) = 1 …(1)
Clearly ae = 2 …(2)
and \(\frac{2 b^2}{a}\) = \(\frac{10}{3}\) ⇒ b² = \(\frac{5}{3}\)a …(3)
We know that b² = a² (1 – e²)
⇒ \(\frac{5a}{3}\) = a² – 4 [using eqn. (2) and (3)]
⇒ 3a² – 5a – 12 = 0
⇒ (a – 3) (3a + 4) = 0
⇒ a = 3 [∵ a > 0]
∴ from (2); e = \(\frac{2}{3}\) < 1
Thus b² = \(\frac{5}{3}\) × 3 = 5 [using (3)]
Hence eqn. (1) reduces to ; \(\frac{x^2}{9}\) + \(\frac{y^2}{5}\) = 1
which is the required eqn. of an ellipse.

Question 27.
Find the eccentricity of the ellipse of minor axis 2b, if the line segment joining the foci subtends an angle 2a at the upper vertex. Also, find the equation of the ellipse.
Solution:
Let e be the eccentricity of an ellipse.
In △BOS, we have OS = b tan α
∴ SS’ = 20S ⇒ 2ae – 2b tan α
⇒ ae = b tan α …(1)

We know that b² = a² (1 – e²)
⇒ b² = a² – a²e² = a² – b² tan² α
⇒ b² (1 + tan² α) = a²
a = b sec α …(2)
From eqn. (1) and eqn. (2); we have
e = \(\frac{b \tan \alpha}{b \sec \alpha}\) = sin α
Thus required eqn. of ellipse becomes ;
\(\frac{x^2}{b^2 \sec ^2 \alpha}\) + \(\frac{y^2}{b^2}\) = 1
⇒ x² cos² α + y² = b²
which is the required eqn.

Students can cross-reference their work with S Chand ISC Maths Class 11 Solutions Chapter 23 Parabola Chapter Test to ensure accuracy.

S Chand Class 11 ICSE Maths Solutions Chapter 23 Parabola Chapter Test

Question 1.
The equation of the directrix of the parabola is 3x + 2y + 1 = 0. The focus is (2, 1). Find the equation of the parabola.
Solution:
Given focus of parabola be F (2,1) and eqn. of directrix be 3x + 2y + 1 = 0.
Let P (x, y) be any point in the plane of cLiiectrix and focus.Let PM be the ⊥ drawn from P on given directrix.

On squaring both sides, we have
(x – 2)² + (y – 1)² = \(\frac{(3 x+2 y+1)^2}{13}\)
⇒ 13 [(x- 2)² + (y – 1)²] = (3x + 2y + 1)²
⇒ 13 (x² – 4x + y² – 2y + 5]
= 9x² + 4y² + 1 + 12xy + 4y + 6x
⇒ 4x² + 9y² – 12xy – 58x – 30y + 64 = 0
which is the required of parabola.

Question 2.
The points (0, 4) and (0, 2) are the vertex and focus of a parabola. Find the equation of the parabola.
Solution:
Given (0, 4) be the vertex and (0, 2) be focus of the parabola.
Since x-coordinate of both points be same. Hence axis of the parabola is y-axis. Further vertex (0, 4) lies above the focus (0, 2).

Thus the parabola be a downward parabola, length of latus rectum = 4a = 4 | VF | =4 × 2 = 8
Thus the required eqn. of parabola be
(x – 0)² = – 4a(y – 4)
⇒ x² = – 8 (y – 4)
⇒ x² + 8y – 32 = 0
which is required eqn. of parabola.

Question 3.
Find the equation of the parabola with latus rectum joining points (4, 6) and (4,-2).
Solution:
Given the ends of latus rectum are L (4, 6) and L’ (4, – 2).
Since the x-coordinates of both points L and L’ are equal.
∴ Latus-rectum is || to y-axis and hence axis of the parabola is parallel to x-axis.
[since axis ⊥ to latus rectum] Thus, the eqns. of two possible parabolas taken as, (y – k)² = ± 4a (x – h) …(1)
where (h, k) be the vertex of the parabolas.

length of latus-rectum = |LL’|
= \(\sqrt{(4-4)^2+(6+2)^2}\) = 8 = 4 a
∴ eqn. (1) reduces to,
(y – k)² = ± 8(x – h) …(2)
Since the point L (4, 6) lies on eqn. (2); we have
(6 – k)² = 8 (4 -h) …(3)
(6 – k)2 = – 8 (4 – h) …(4)
Also, the point L’ (4, – 2) lies on eqn. (2); we have

On dividing (3) and (5); we have
\(\frac{(6-k)^2}{(2+k)^2}\) = 1 ⇒ (6 – k)² = (2 + k)²
⇒ 36 + k² – 12k = k² + 4k + 4
⇒ 16k = 32 ⇒ k = 2
∴ from (5) ; (- 2 – 2)² = 8 (4 -h)
⇒ 16 = 8 (4 – h) ⇒ 2 = 4 – h ⇒ h = 2
Hence vertex of parabola be A (2, 2).
Thus eqn. of parabola be
(y – 2)² = 8(x – 2)
⇒ y² – 4y – 8x + 20 = 0
On dividing eqn. (4) and (6); we have
k = 2 ∴ from (4); 16 = -8(4 – h)
⇒ – 2 = 4 – h ⇒ h = 6
Thus vertex of required parabola be A’ (6, 2)
∴ eqn. of required parabola be given by (y – 2)² = – 8(x – 6)
⇒ y² – 4y + 8x – 44 = 0

Question 4.
Find the equation of the parabola whose focus is (- 1, – 2) and the equation of the directrix is given 4x – 3y + 2 = 0. Also find the equation of the axis.
Solution:
Given focus of parabola be F (- 1, – 2) and eqn. of directrix be

Let P (x, y) be any point in the plane of directrix and focus. Let PM be the 1 drawn from P (x, y) on given directrix. Then P (x, y) lies on parabola iff PF = PM

⇒ 25 [x² + 2x + y² + 4y + 5] = 16x² + 9y² + 4 – 24xy – 12y + 16x
⇒ 9x² + 16y² + 24xy + 34x + 112y + 121 = 0
which is the required eqn. of parabola. Since axis be the line ⊥ to directrix and passing through the focus of parabola, eqn. of line ⊥ to eqn. (1) be
3x + 4y + k = 0 …(2)
and eqn. (2) passes through the focus F (- 1, – 2)
∴ – 3 – 8 + k = 0
⇒ k = 11
∴ from (2); 3x + 4y + 11 = 0 be the required eqn. of axis of parabola.

Question 5.
Find the equation of the parabola if its vertex is at (0, 0), passes through (5, 2) and is symmetric w.r.t. y-axis.
Solution:
We know that a curve F (x, y) = 0 be symmetric w.r.t. y-axis if
F (-x, y) = F (x, y)
Let the eqn. of parabola having vertex (0, 0) and symmetric w.r.t. y be (x – 0)² = 4a(y – 0)
i.e. x² = 4ay …(1)
it passes through the point (5, 2); we have
25 = 8a ⇒ a = \(\frac { 25 }{ 8 }\)
Thus eqn. (1) reduces to ;
x² = 4 × \(\frac { 25 }{ 8 }\)y = \(\frac { 25 }{ 2 }\)y
which is the required eqn. of parabola.

Question 6.
The parabola y² = 4ax passes through the point (2, – 6). Find the length of its latus rectum.
Solution:
Given eqn. of parabola be
y² = 4ax …(1)
Now eqn. (1) passes through the point (2, – 6).
∴ 36 = 4a × 2 ⇒ a = \(\frac { 36 }{ 8 }\) = \(\frac { 9 }{ 2}\)
∴ length of latus rectum = 4a = 4 × \(\frac { 9 }{ 2}\) = 18.

Question 7.
Find the coordinates of the vertex and the focus of the parabola y² = 4(x + y).
Solution:
Equation of given parabola be
y² = 4 (x + y)
⇒ y² – 4y = 4x
⇒ y² – 4y + 4 = 4x + 4
⇒ (y – 2)² = 4 (x + 1) …(1)
shifting the origin to point (- 1, 2),
putting x + 1 =X and y – 2 = Y in eqn. (1); we have
Y² = 4X …(2)
Here 4a – 4
⇒ a = 1
which represents a right handed parabola with axis x-axis.
Vertex of parabola (2) be given by
X = 0; Y = 0
x + 1 = 0 and y – 2 = 0
i.e. x = – 1 and y = 2
Thus (-1, 2) be the vertex of parabola (1). Further, focus of parabola (2) be given by
X = a and Y = 0
⇒ x + 1 = 1 and y – 2 = 0
⇒ x = 0 and y = 2
Thus (0, 2) be the focus of parabola (1).

Question 8.
Find the focus, the equation of the directrix and the length of the latus rectum of the parabola y² + 12 = 4x+ 4y.
Solution:
Given eqn. of parabola be
y² – 4x – 4y + 12 = 0
⇒ y² – 4y = 4x – 12
⇒ y² – 4y + 4 = 4x – 8
⇒ (y – 2)² = 4 (x – 2) …(1)
Shifting the origin to point (2, 2) and putting
x – 2 = X and y – 2 = Y in eqn. (1);
we get Y² = 4X …(2)
which represents a right handed parabola.
On comparing with Y² = 4aX
∴ length of latus rectum = 4a = 4 units
Focus of eqn. (2) be given by
X = a, Y = 0
⇒ x – 2 = 1 and y – 2 = 0
i.e. x = 3 and y = 2
Thus (3, 2) are the coordinates of focus of eqn. (1).
eqn. of directrix of parabola (2) be given by X = – a ⇒ x – 2 = – 1 ⇒ x = 1
which is the eqn. of directrix.

Effective S Chand ISC Maths Class 11 Solutions Chapter 23 Parabola Ex 23 can help bridge the gap between theory and application.

S Chand Class 11 ICSE Maths Solutions Chapter 23 Parabola Ex 23

Question 1.
The focus at (10, 0) the directrix x = -10.
Solution:
Given focus is at (10, 0)
∴ axis of parabola along x-axis and its eqn. can be taken as y² = 4ax [Here a = 10]
∴ y² = 40x be the required eqn. of parabola.

Question 2.
The focus at (0, 5), the directrix y = – 5.
Solution:
Given focus is at (0, 5), on comparing with (0, a) ∴ a = 5

Thus axis of parabola is along y-axis.
Hence required eqn. of parabola be x² = (4 × 5)y i.e. x² = 20y

Question 3.
The focus at (- 3,0), the directrix x + 5 = 0.
Solution:
Let P (x, y) be any point on the parabola.
Then by definition, we have
| PF | = | PM |
\(\sqrt{(x-2)^2+(y+3)^2}\) = \(\frac{|x+5|}{1}\)
On squaring both sides ; we have
(x + 3)² + y² = (x + 5)²
⇒ x² + 6x + 9 + y² = x² + 10x + 25
⇒ y² = 4x + 16 = 4 (x + 4)

Question 4.
The focus at (2, – 3), the directrix x + 5 = 0.
Solution:
Given focus be F (2, – 3) and eqn. of directrix be x + 5 = 0
Let P (x, y) be any point on parabola.
\(\sqrt{(x-2)^2+(y+3)^2}\) = \(\frac{|x+5|}{1}\)
On squaring both sides ; we have
(x – 2)² + (y + 3)² = (x + 5)²
⇒ x² – 4x + 4 + y² + 6y + 9 = x² + 10x + 25
⇒ y² – 14x + 6y – 12 = 0,
which is the required eqn. of parabola.

Question 5.
The focus at (1, 1), the directrix x – y = 3.
Solution:
Given focus be F (1, 1) and eqn. of given
directrix be x-y -3 = 0
Let P (x, y) be any point on parabola
then by def. | PF | = | PM |
\(\sqrt{(x-1)^2+(y-1)^2}\) = \(\frac{|x-y-3|}{\sqrt{1^2+(-1)^2}}\)
On squaring both sides ; we have
(x – 1)² + (y – 1)² = \(\frac{|x-y-3|^2}{2}\)
⇒ 2 [(x – 1)² + (y – 1)²] = (x – y – 3)²
⇒ 2x² + 2y² – 4x – 4y + 4 = x² – y² + 9 – 2xy + 6y – 6x
⇒ x² + y² + 2xy + 2x – 10y – 5 = 0
which is the required eqn. of parabola.

Question 6.
The vertex at the origin, the axis along the x-axis, and passes through (- 3, 6).
Solution:
Let the eqn. of parabola whose vertex is at (0,0) and axis along x-axis be given by
y² = 4ax …(1)
eqn. (1) passes through the point (-3,6)
∴ 36 = 4a (- 3) ⇒ a = – 3
Thus eqn. (1) reduces to ; y² = – 12x be the required eqn. of parabola.

Question 7.
The focus at (- 2, -1) and the latus rectum joins the points (- 2, 2) and (- 2, – 4).
Solution:
Given focus of required parabola be (- 2, -1) and end points of latus rectum are (- 2, 2) and (- 2, – 4)
since x-coordinates of both end points be same.
∴ axis of the parabola be parallel to x-axis Let the eqn. of parabola be taken as :
(y – α)² = ± 4a (x – β) …(1)
length of latus rectum = 4a
= \(\sqrt{(-2+2)^2+(-4-2)^2}\) = 6
Thus eqn. (1) reduces to ;
(y – α)² = ± 6 (x – β) …(2)
The points (- 2, 2) and (- 2, – 4) lies on eqn. (2); we get
(2 – α)² = ± 6 (- 2 – β) …(3)
(- 4 – α)² = ± 6 (- 2 – β) …(4)

On dividing (3) by (4); we have
(2 – α)² = (a + 4)²
⇒ α² – 4α + 4 = α² + 8α + 16
⇒ α = – 1
∴ from (3); 9 = ± 6 (- 2 – α)
⇒ \(\frac { 3 }{ 2 }\) = ± (- 2 – β)
Case-I. \(\frac { 3 }{ 2 }\) = (- 2 – β)
⇒ β = – 2 – \(\frac { 3 }{ 2 }\) = –\(\frac { 7 }{ 2 }\)

Case-II. \(\frac { 3 }{ 2 }\) = – (- 2 – β)
⇒ \(\frac { 3 }{ 2 }\) = 2 + β ⇒ β = – \(\frac { 1 }{ 2 }\)
∴ from (1); we have
(y + 1)² = + 6\(\left(x+\frac{7}{2}\right)\)
⇒ y² + 2y – 6x – 20 = 0
and (y + 1)² = – 6\(\left(x+\frac{1}{2}\right)\)
⇒ y² + 2y – 6x + 4 = 0

Question 8.
The vertex at (- 2,3) and the focus at (1,3).
Solution:
Clearly the axis of the parabola be parallel to x-axis and its eqn. can be taken as,
(y – β)² = 4a (x – α) …(1)

given vertex of parabola be (- 2, 3).
∴ eqn. (1) reduces to
(y – 3)² = 4a (x + 2) …(2)
a – distance between focus and vertex
= 1 – (- 2) = 3
∴ eqn. (2) reduces to ;
(y – 3)² = 12 (x + 2)
⇒ y² – 6y – 12x – 15 = 0
which is the required eqn. of parabola.

Question 9.
The vertex at (0, 0) and the focus at (0, 1).
Solution:
Given vertex of parabola be V (0, 0) and focus be F (0, 1).

Thus the required eqn. of parabola be
x² = 4ay …(1)
since axis of parabola bey-axis, distance between focus and vertex = a = 1 – 0 = 1
∴ eqn, (1) reduces to ;
x² = 4y be the required eqn. of parabola.

Question 10.
The vertex at (0, a) and the focus at (0,0).
Solution:
Given vertex of parabola be V (0, a) and Focus F (0, 0).
Thus y-axis be the axis of downward parabola.

Hence required eqn. of parabola having vertex (0, a) be given by
(x – 0)² = – 4a (y – a)
⇒ x² = 4a(a – y)

Question 11.
The axis parallel to the x-axis, and the parabola passes through (3, 3), (6, 5) and (6, – 3).
Solution:
Let the eqn. of parabola having axis parallel to x-axis be given by
x = ay², + by + c …(1)
where a, b, c are constants,
eqn. (1) passes through the points (3, 3), (6, 5) and (6, – 3).
3 = 9a + 3b + c …(2)
6 = 25a + 5 b + c …(3)
6 = 9a – 3b + c …(4)
eqn. (2) – eqn. (4) gives ;
– 3 = 6b ⇒ b = –\(\frac { 1 }{ 2 }\)
∴ from (2); \(\frac { 9 }{ 2 }\) = 9a + c …(5)
\(\frac { 17 }{ 2 }\) = 25a + c …(6)
eqn. (6) – (5) gives;
\(\frac { 17 }{ 2 }\) – \(\frac { 9 }{ 2 }\) = 16 a
⇒ a = \(\frac { 1 }{ 4 }\)
∴ from (5); c = \(\frac { 9 }{ 2 }\) – \(\frac { 9 }{ 4 }\) = \(\frac { 9 }{ 4 }\)
∴ from (1); x = \(\frac { 1 }{ 4 }\) y² – \(\frac { 1 }{ 2 }\)y + \(\frac { 9 }{ 4 }\)
⇒ y² – 2y – 4x + 9 = 0
which is the required eqn. of Parabola.

Question 12.
The axis parallel to the y-axis and the parabola passes through the points (4, 5), (-2, 11) and (-4, 21).
Solution:
The equation of the parabola whose axis is parallel to y-axis can be taken as
y = Ax² + Bx + C …(1)
where A, B and C are arbitrary constants. Since the parabola (1) passes through the point A (4, 5).
5 = 16A + 4B + C …(2)
Clearly the points (-2, 11) and (-4, 21) lies on eqn. (1).
11 = 4A – 2B + C …(3)
21 = 16A – 4B + C …(4)
eqn. (4) – eqn. (2); we have
16 = – 8B ⇒ B = – 2
eqn. (2) – eqn. (3) gives ;
– 6 = 12A + 6B ⇒ – 6 = 12A – 12
⇒ A = \(\frac { 1 }{ 2 }\)
∴ from (2); 5 = 8 – 8 + C ⇒ C = 5
putting the values of A, B and C in eqn. (1); we have
y = \(\frac{x^2}{2}\) – 2x + 5
⇒ x² – 4x – 2y + 10 = 0
be the required eqn. of parabola.

Question 13.
The parabola y² = 4px passes through the point (3, – 2). Obtain the length of the latus rectum and the coordinates of the focus.
Solution:
Given eqn. be parabola be
y² = 4px …(1)
since eqn. (1) passes through the point (3, – 2)
∴ 4 = 4p × 3 ⇒ 12p = 4 ⇒ P = \(\frac { 1 }{ 3 }\)
∴ eqn. of parabola becomes
y² = \(\frac { 4 }{ 3 }\)x …(2)
On comparing with y² = 4ax
So eqn. (2) represents right handed parabola.
Here, 4a = \(\frac { 4 }{ 3 }\) ⇒ a = \(\frac { 1 }{ 3 }\)
∴ focus of parabola be (a, 0) i.e. \(\left(\frac{1}{3}, 0\right)\)
and length of latus-rectum = 4a = \(\frac { 4 }{ 3 }\)
∴ focus of parabola be (a, 0) i.e. \(\left(\frac{1}{3}, 0\right)\)
and length of latus-rectum = 4a = \(\frac { 4 }{ 3 }\)

Question 14.
Prove that the equation y² + 2ax + 2by + c = 0 represents a parabola whose axis is parallel to the axis of x. Find its vertex.
Solution:
Given eqn. be,
y² + 2ax + 2by + c = 0
⇒ y² + 2by + b² – b² + 2ax + c = 0
⇒ (y + b)² = – 2ax + b² – c
⇒ (y + b)² = -2a\(\left[x-\frac{b^2-c}{2 a}\right]\) …(1)
As it is of the form (y – β)² = 4a (x – a)
Thus eqn. (1) represents a parabola whose axis parallel to x-axis with vertex \(\left(\frac{b^2-c}{2 a},-b\right)\)

Question 15.
Of the parabola, 4(y – 1)² = – 7 (x – 3) find
(i) the length of the latus rectum.
(ii) the coordinates of the focus and the vertex.
Solution:
Given eqn. of parabola be
4(y – 1)² = – 7 (x – 3)
⇒ (y – 1)² = –\(\frac { 7 }{ 4 }\)(x – 3) …(1)
which is clearly represents a parabola with axes parallel to x-axis with vertex (3, 1)
putting y – 1 = Y and x – 3 = X
eqn. (1) reduces to ; Y² = –\(\frac { 7 }{ 3 }\)X,
Here 4a = \(\frac { 7 }{ 4 }\) ⇒ a = \(\frac { 7 }{ 16 }\)
∴Focus is given by (- a, 0)
i.e. X = – a and Y = 0
⇒ x – 3 = –\(\frac { 7 }{ 16 }\) and y – 1 = 0
⇒ x = \(\frac { 41 }{ 16 }\) and y = 1
i.e. focus of parabola given by (1) be \(\left(\frac{41}{16}, 1\right)\)

Question 16.
Find the vertex, focus, and directrix of the following parabolas :
(i) y² – 2y + 8x – 23 = 0
(ii) x² + 8x + 12y + 4 = 0
Solution:
(i) Given eqn. of parabola be
y² – 2y + 8x – 23 = 0
⇒ y² – 2y + 1 = – 8x + 23 + 1
⇒ (y – 1)² = – 8 (x – 3) …(1)
Transferring origin to point (3,1); we put x-3 = X; y – 1 = Y in eqn. (1); we get Y² = – 8X, which is a left handed parabola, on comparing with Y² = – 4aX i.e. 4a = 8 ⇒ a = 2
Thus, vertex is given by X = 0, Y = 0
i.e. x – 3 = 0 and y – 1 = 0
i.e. x = 3, y = 1
Thus vertex of eqn. (1) be given by (3,1). and Focus is given by X = – a, Y = 0
i.e. x – 3 = – 2 and y – 1 = 0
i.e. x = 1 and y = 1
Thus focus of eqn. (1) be given by (1, 1) and directrix is given by X = a
⇒ x – 3 = 2 ⇒ x – 5 = 0 bethe required eqn. of directrix.

(ii) Given eqn. of parabola be
x² + 8x + 12y + 4 = 0
⇒ (x2 + 8x + 16) + 12y + 4 – 16 = 0
⇒ (x + 4)² = – 12 (y – 1) …(1)
Shifting the origin to point (- 4, 1)
we put x + 4 = X; y – 1 = Y in eqn. (1).
X² = – 12Y …(2)
which represents a downward parabola.
On comparing eqn. (2) with X² = – 4aY
∴ 4a = 12 ⇒ a = 3
Thus vertex of eqn. (2) be given by
X = 0, Y = 0
i.e. x + 4 = 0 and y – 1 = 0
i.e. x = -4 and y = 1
Thus (- 4,1) be the vertex of parabola (1) Focus of eqn. (2) be given by
X = 0 and Y = – a
⇒ x + 4 = 0 and y – 1 = – 3
i.e. x = -4 and y = -2
Hence (-4, -2) be the focus of parabola (1). Thus, directrix of the parabola (2) be given by
Y = a i.e. y – 1 = 3 ⇒ y = 4
be the required directrix of parabola (1).

Question 17.
Find the vertex, focus, and directrix of the parabola (x – h)² + 4a(y – k) = 0.
Solution:
Given eqn. of parabola be
(x – h)² = – 4a(y – K) …(1)
Shifting (0, 0) to point (h, k)
putting x – h = X
and y – k = Y in eqn. (1); we have
X² = – 4aY …(2)

which represents a downward parabola vertex of eqn. (2) be given by X = 0 = Y
i.e. x – h = 0 = y – k i.e. x = h and y = k
Thus (h, k) be the vertex of parabola (1).
Focus of (2) be given by X = 0 and Y = -a
i.e. x – h – 0 and y – k = – a
i.e. x = h and y = k – a
Thus (h, k – a) be the required focus of parabola (1) and directrix of parabola (2)
be given by Y = a ⇒ y – k = a ⇒ y = k + a which is required eqn. of directrix of parabola (1).

Question 18.
Find the equation to the parabola whose axis is parallel to the y-axis and which passes through the points (0, 4), (1, 9) and (- 2, 6) and determine its latus rectum.
Solution:
Let the eqn. of parabola whose axes parallel to y-axis be given by
y = ax² + bx + c …(1)
where a, b, c are all constants
eqn. (1) passes through the points (0, 4), (1, 9) and (-2, 6).
4 = a × 0² + b × 0 + c ⇒ c = 4
9 = a + b + c ⇒ a + b = 5 ..(2)
6 = 4a – 2b + c ⇒ 4a – 2b = 2
⇒ 2a – b = 1 …(3)
On adding (2) and (3) ; we have
3a = 6 ⇒ a = 2 ∴ b = 3
putting the values a, b and c in eqn. (1); we have
y = 2x² + 3x + 4
⇒ y = 2 \(\left(x^2+\frac{3}{2} x+\frac{9}{16}\right)\) – \(\frac { 9 }{ 16 }\) + 4
⇒ y = 2 \(\left(x+\frac{3}{4}\right)^2\) + \(\frac { 55 }{ 16 }\)
⇒ \(2\left(x+\frac{3}{4}\right)^2\) = \(\left(y-\frac{55}{16}\right)\)
⇒ \(\left(x+\frac{3}{4}\right)^2\) = \(\frac { 1 }{ 2 }\)\(\left(y-\frac{55}{16}\right)\)
∴ length of latus rectum = \(\frac { 1 }{ 2 }\)

Question 19.
Find the coordinates of the point on the parabola y² = 8x whose focal distance is 8.
Solution:
Given eqn. of parabola be y² = 8x …(1)
Let (x₁, y₁) be any point on curve (1).
∴ y₁² = 8x₁ …(2)
On comparing eqn. (1) with y² = 4ax
∴ 4a = 8 ⇒ a = 2
Also, given focal distance = 8 ⇒ x_i + a = 8
⇒ x₁ = 8 – 2 = 6
∴ from (2); y₁² = 8 × 6 = 48
⇒ y₁ = ±4√3
Hence the required point on given parabola be (6, ± 4√3).

Question 20.
If the ordinate of a point on the parabola y² = 4ax is twice the latus rectum, prove that the abscissa of this point is twice the ordinate.
Solution:
Given eqn. of parabola be
y² = 4ax …(1)
Let (x₁, y₁) be any point on parabola (1).
∴ y₁² = 4ax₁ …(2)
given y₁ = 2 × length of latus rectum
= 2 × 4a = 8a
∴ from (2); (8a)² = 4ox₁
⇒ x₁ = \(\frac{64 a^2}{4 a}\) = 16a ⇒ x₁ = 2 (8a) = 2y₁
Thus abscissa of this point is twice the ordinate.

Question 21.
Find the equation of the parabola whose focus is at the origin, and whose directrix is the line y – x = 4. Find also the length of the latus rectum, the equation of the axis, and the coordinates of the vertex.
Solution:
Given (0, 0) be the focus of required parabola whose directrix is the line
y – x – 4 = 0
Let P (x, y) be any point on parabola.
Then by def. | PF | = | PM |

On squaring both sides ; we have
2 [x² + y²] = (y – x – 4)²
⇒ 2(x² + y²) = y² + x² – 2xy + 8x – 8y + 16
⇒ x² + y² + 2xy – 8x + 8y – 16 = 0
which is the required eqn. of parabola, length of latus rectum = 4a = 2 × 2a
= 2 (length of ⊥ drawm from focus (0, 0) to directrix y – x – 4 = 0)

Clearly axis be the line through the focus F (0, 0) and ⊥ to directrix
y – x – 4 = 0 …(1)
eqn. of axis be given by
x + y + k = 0
it pass through F (0, 0). ∴ k = 0
Thus, x + y = 0 …(2)
be the eqn. of axis of parabola.
Let Z be the point of intersection of directrix and axis of parabola. So we solve eqn. (1) and eqn. (2) simultaneously ; we have
y = 2 and x = – 2
∴ Coordinates of point Z are (- 2, 2). and vertex (α, β) be the mid-point of FZ.
α = \(\frac{-2+0}{2}\) and β = \(\frac{2+0}{2}\)
i.e. a = – 1 and β = 1
Thus, required vertex of parabola be (-1, 1).

Question 22.
The directrix of a conic section is the straight line 3x – 4y + 5 = 0 and the focus is (2, 3). If the eccentricity e is 1, find the equation to the conic section. Is the conic section a parabola ?
Solution:
Since eccentricity of conic section be 1.
∴ conic section is a parabola,
given eqn. of directrix be 3x – 4y + 5 = 0 and focus be F (2, 3). Let P (x, y) be any point on parabola.
Then by def. | PF | = | PM |

On squaring both sides ; we have
25 [(x – 2)² + (y – 3)²] = (3x – 4y + 5)²
⇒ 25 [x² + y² – 4x – 6y + 13]
= 9x² + 16y² + 25 – 24xy – 40y + 30x
⇒ 16x² + 9y² + 24xy – 130x – 110y + 300 = 0
which is the required eqn. of parabola.

Question 23.
Find the equation to the parabola whose focus is (-2, 1) and directrix is 6x – 3y = 8.
Solution:
Given focus be (- 2, 1) and eqn. of directrix be 6x – 3y – 8 = 0
Let P (x, y) be any point on parabola.
Then by def. | PF | = | PM |

On squaring both sides ; we have
45 [(x + 2)² + (y – 1)²] = (6x -3y- 8)²
⇒ 45 [x² + 4x + y² – 2y + 5]
= 36x² + 9y² + 64 – 36xy + 48y – 96x
⇒ 9x² + 36y² + 36xy + 276x – 138y + 161 = 0
which is the required eqn. of parabola.

Question 24.
The length of the latus rectum of the parabola whose focus is (3, 3) and directrix is 3x – 4y – 2 = 0 is
(a) 2
(b) 1
(c) 4
(d) None of these
Solution:
Required length of latus rectum = 4a
= 2 × (length of 1 draw from focus (3, 3) to directrix 3x – 4y – 2 = 0)

Continuous practice using ISC OP Malhotra Solutions Class 11 Chapter 19 Differentiation Chapter Test can lead to a stronger grasp of mathematical concepts.

S Chand Class 11 ICSE Maths Solutions Chapter 19 Differentiation Chapter Test

Question 1.
Find from first principles the differential coefficient of 2x² + 3x.
Solution:
Let y =f (x) = 2x² + 3x
∴ f(x + δx) = 2 (x + δx)² + 3 (x + δx)
Then by first principle, we have

Question 2.
Find from first principles the differential coefficient of sin 2x.
Solution:
Let y = f(x) = sin 2x
∴ f(x + δx) = sin 2 (x + δx)
Then by first principle, we have

Question 3.
f(x) = \(\sqrt{3 x+4}\), x > – 1
Solution:
Given f(x) = \(\sqrt{3 x+4}\); Diff. both sides w.r.t. x, we have
\(\frac{d}{d x}\) f(x) = f ‘ (x) = \(\frac{d{d x}\) (3x + 4)^1/2

Question 4.
f(x) = \(\sqrt{4-x}\), x < 4
Solution:
Given f(x) = \(\sqrt{4-x}\); Diff. both sides w.r.t. x, we have

Question 5.
f(x) = \(\frac{3 x+4}{4 x+3}\left(x \neq \frac{-3}{4}\right)\)
Solution:
Given f(x) = \(\frac{3 x+4}{4 x+3}\); Diff. both sides w.r.t. x, we have

Question 6.
f(x) = \(\sqrt{x^2+1}\)
Solution:
Given f(x) = \(\sqrt{x^2+1}\); Diff. both sides w.r.t. x, we have

Question 7.
(2x + 3) (x² – x + 2)
Solution:
Let Y = (2x + 3) (x² – x + 2); Diff. both sides w.r.t. x, we have
\(\frac{dy}{d x}\) = (2x + 3) \(\frac{d}{d x}\) (x² – x + 2) + (x² – x + 2) \(\frac{d}{d x}\) (2x + 3)
[∵\(\frac{d}{d x}\) (uv) = \(\frac{udv}{d x}\) + v\(\frac{du}{d x}\)]
= (2x + 3) (2x – 1) + (x² – x + 2) . 2 = 4x² + 4x – 3 + 2x² – 2x + 4 = 6x² + 2x + 1

Question 8.
tan (5x + 7)
Solution:
Let y = tan (5x + 7); Diff. both sides w.r.t. x, we have
\(\frac{dy}{d x}\) = \(\frac{d}{d x}\) tan (5x + 7) = sec² (5x + 7)
= sec² (5x + 7) [5.1 + 0] = 5 sec² (5x + 1)

Question 9.
sin² (3x – 2)
Solution:
Let y = sin² (3x – 2); Diff. both sides w.r.t. x, we have
\(\frac{dy}{d x}\) = \(\frac{d}{d x}\) [sin (3x – 2)]² = 2 sin (3x – 2) \(\frac{d}{d x}\) sin (3x – 2)
= 2 sin (3x – 2) cos (3x – 2) . 3 = 3 sin 2 (3x – 2)
= 3 sin (6x – 4)

Question 10.
(x³ + sin x)⁵
Solution:
Let y = (x³ + sin x)⁵; Diff. both sides w.r.t. x, we have
\(\frac{dy}{d x}\) = 5 (x³ + sin x)⁴ \(\frac{d}{d x}\) (x³ + sin x) = 5 (x³ + sin x)⁴ (3x² + cos x)

Peer review of ISC OP Malhotra Solutions Class 11 Chapter 19 Differentiation Ex 19(d) can encourage collaborative learning.

S Chand Class 11 ICSE Maths Solutions Chapter 19 Differentiation Ex 19(d)

Question 1.
sin 5x
Solution:
Let y – sin 5x ; diff. both sides w.r.t. x, we have
\(\frac{d y}{d x}\) = \(\frac{d}{d x}\) sin 5x = cos 5x\(\frac{d}{d x}\)(5x) = 5 cos 5x

Question 2.
cos 8x
Solution:
Let y = cos 8x ; Diff. both sides w.r.t. x, we have
\(\frac{d y}{d x}\) = \(\frac{d}{d x}\) (cos 8x) = -sin8x\(\frac{d}{d x}\) (8x) = – 8 sin x

Question 3.
sin (5x + 9)
Solution:
Let y = sin (5x + 9); Diff. both sides w.r.t. x,
\(\frac{d y}{d x}\) = \(\frac{d}{d x}\) sin (5x + 9) = cos(5x + 9)\(\frac{d}{d x}\)(5x + 9) = cos (5x + 9) (5.1 + 0) = 5 cos (5x + 9)

Question 4.
cos (2x – 3)
Solution:
Let y = cos (2x – 3) ; diff. both sides w.r.t. x, we have
\(\frac{d y}{d x}\) = \(\frac{d}{d x}\)cos(2x – 3) = -sin(2x – 3)\(\frac{d}{d x}\)(2x – 3) = – 2 sin (2x – 3)

Question 5.
tan 7x
Solution:
Let y = tan 7x ; Diff. both sides w.r.t. x, we have
\(\frac{d y}{d x}\) = \(\frac{d}{d x}\) tan 7x = sec² 7x\(\frac{d}{d x}\) (7x) = 7 sec² 7x

Question 6.
cot nx
Solution:
Let y = cot nx ; Diff. both sides w.r.t. x, we get
\(\frac{d y}{d x}\) = \(\frac{d}{d x}\) cot nx = – cosec² nx\(\frac{d}{d x}\) (nx) = – n cosec² nx

Question 7.
tan (6x + 11)
Solution:
Let y = tan (6x + 11); Diff. both sides w.r.t. x
\(\frac{d y}{d x}\) = sec²(6x +11) \(\frac{d}{d x}\)(6x + 11) = sec² (6x + 11) (6.1 + 0) = 6 sec² (6x + 11)

Question 8.
sin \(\frac{x}{3}\)
Solution:
Let y = sin\(\frac{x}{3}\); diff. both sides w.r.t. x, we have
\(\frac{d y}{d x}\) = cos\(\frac{x}{3}\) \(\frac{d}{d x}\) \(\frac{x}{3}\) = \(\frac{1}{3}\) cos \(\frac{x}{3}\)

Question 9.
sec mx
Solution:
Let y = sec mx ; diff. both sides w.r.t. x, we have
\(\frac{d y}{d x}\) = \(\frac{d}{d x}\) (sec mx) = sec mx tan mx \(\frac{d}{d x}\) (mx) = m sec mx tan mx

Question 10.
Solution:
Let y = sec \(\left(\frac{x}{2}-1\right)\); Diff. both sides w.r.t. x, we get
\(\frac{d y}{d x}\) = \(\frac{d}{d x}\) sec\(\left(\frac{x}{2}-1\right)\) = sec\(\left(\frac{x}{2}-1\right)\) tan \(\left(\frac{x}{2}-1\right)\) \(\frac{d}{d x}\) \(\left(\frac{x}{2}-1\right)\) = \(\frac{1}{2}\) sec \(\left(\frac{x}{2}-1\right)\) tan \(\left(\frac{x}{2}-1\right)\)

Question 11.
cosec \(\frac{2}{3}\)x
Solution:
Let y = cosec \(\frac{2}{3}\)x ; Diff. both sides w.r.t. x, we get
\(\frac{d y}{d x}\) = – cot\(\frac{2}{3}\) x cosec \(\frac{2}{3}\)x \(\frac{d}{dx}\) \(\left(\frac{2}{3} x\right)\) = –\(\frac{2}{3}\) cot \(\frac{2}{3}\) x cosec \(\frac{2x}{3}\)

Question 12.
x sin x
Solution:
Let y = x sin x; Diff. both sides w.r.t. x, we get
\(\frac{d y}{d x}\) = \(\frac{d}{d x}\) (x sin x) = x\(\frac{d}{d x}\) sin x + sin x \(\frac{d}{d x}\)(x)
[∵ \(\frac{d}{d x}\) (uv) = u\(\frac{dv}{d x}\) + v \(\frac{du}{d x}\)]
= x cos x + sin x . 1 = x cos x + sin x

Question 13.
x² cos 5x
Solution:
Let y = x² cos 5x; Diff. both sides w.r.t. x, we get
\(\frac{d y}{d x}\) = x² \(\frac{d}{d x}\) cos 5x + cos 5x \(\frac{d}{d x}\)x²
[∵ \(\frac{d}{d x}\) (uv) = u\(\frac{dv}{d x}\) + v \(\frac{du}{d x}\)]
= x² (-sin 5x) \(\frac{d}{d x}\) (5x) + cos 5x . 2x = – 5x² sin 5x + 2x cos 5x

Question 14.
\(\sqrt{x}\) cosec (5x + 7)
Solution:
Let y = \(\sqrt{x}\) cosec (5x + 7); Diff. both sides w.r.t. x, we have
\(\frac{d y}{d x}\) = \(\sqrt{x}\) \(\frac{d}{d x}\)cosec (5x + 7) + cosec (5x + 7) \(\frac{d}{d x}\) = \(\sqrt{x}\)
= \(\sqrt{x}\) {- cot (5x + 7) cosec (5x + 7)} \(\frac{d}{d x}\) (5x + 7) + cosec (5x + 7) \(\frac{1}{2} x^{\frac{1}{2}-1}\)
= -5\(\sqrt{x}\) cot (5x + 7) cosec (5x + 7) + \(\frac{1}{2 \sqrt{x}}\) cosec (5x + 7)

Question 15.
\(\frac{\sin 3 x}{x-6}\)
Solution:
Let y = \(\frac{\sin 3 x}{x-6}\); Diff. both sides w.r.t. x, we have

Question 16.
\(\frac{\cos x}{5 x}\)
Solution:

Question 17.
\(\frac{\tan x}{2 x+3}\)
Solution:
Let y = \(\frac{\tan x}{2 x+3}\); Diff. both sides w.r.t. x, we have

Question 18.
\(\frac{\sec (a x-b)}{x^2-2}\)
Solution:
Let y = \(\frac{\sec (a x-b)}{x^2-2}\); diff. both sides w.r.t. x, we have

Question 19.
sin 2x
Solution:
Let y = sin 2x; Diff. both sides w.r.t. x, we have
\(\frac{d y}{d x}\) = \(\frac{d }{d x}\) sin 2x = cos 2x \(\frac{d}{d x}\) (2x) = 2 cos 2x

Question 20.
cos 3x
Solution:
Let y = cos 3x; diff. both sides w.r.t. x, we have
\(\frac{d y}{d x}\) = \(\frac{d }{d x}\) (cos 3x) = – sin 3x\(\frac{d }{d x}\) (3x) = – 3 sin 3x

Question 21.
tan 2x
Solution:
Let y = tan 2x; diff. both sides w.r.t. x, we have
\(\frac{d y}{d x}\) = \(\frac{d }{d x}\) (tan 2x) = sec² 2x\(\frac{d }{d x}\) (2x) = 2 sec² 2x

Question 22.
sin\(\frac{x}{2}\)
Solution:
Let y = sin\(\frac{x}{2}\); diff. both sides w.r.t. x, we have
\(\frac{d y}{d x}\) = \(\frac{d }{d x}\) sin \(\frac{x}{2}\) = cos\(\frac{x}{2}\) \(\frac{d}{dx}\) \(\frac{x}{2}\) = \(\frac{1}{2}\) cos \(\frac{x}{2}\)

Question 23.
sec ax
Solution:
Let y = sec ax ; diff. both sides w.r.t. x, we get
\(\frac{d y}{d x}\) = \(\frac{d }{d x}\) (sec ax) = sec ax tan ax \(\frac{d }{d x}\)(ax) = a sec ax tan ax

Question 24.
sec (px + q)
Solution:
Let y = sec (px + q); diff. both sides w.r.t. x, we get
\(\frac{d y}{d x}\) = \(\frac{d }{d x}\) = sec(px + q) = sec (px + q) tan (px + q) \(\frac{d }{d x}\) (px + q) = p sec (px + q) tan (px + q)

Question 25.
tan (4x – 7)
Solution:
Let y = tan (4x – 7) ; diff. both sides w.r.t. x,
\(\frac{d y}{d x}\) = \(\frac{d }{d x}\) tan(4x – 7) = sec²(4x – 7) \(\frac{d }{d x}\)(4x – 7)
= sec² (4x – 7) (4 × 1 – 0) = 4 sec² (4x – 7)

Regular engagement with ISC OP Malhotra Solutions Class 11 Chapter 19 Differentiation Ex 19(c) can boost students’ confidence in the subject.

S Chand Class 11 ICSE Maths Solutions Chapter 19 Differentiation Ex 19(c)

Question 1.
(ax + b) (cx + d)
Solution:
Let y = (ax + b) (cx + d)
Diff. both sides w.r.t. x, we have
\(\frac { dy }{ dx }\) = (ax + b)\(\frac { d }{ dx }\)(cx + d) + (cx + d)\(\frac { d }{ dx }\)(ax + b)
[∵ \(\frac { d }{ dx }\) (uv) = u\(\frac { dv }{ dx }\) + v\(\frac { du }{ dx }\)]
= (ax + b) (c1 + 0) + (cx + d) (a1 + 0)
= c (ax + b) + a (ax + d)

Question 2.
(x¹⁰⁰ + 2x⁵⁰ – 3) (7x⁸ + 20x + 5)
Solution:
Let y = (x¹⁰⁰ + 2x⁵⁰ – 3) (7x⁸ + 20x + 5)
Diff. both sides w.r.t. x, we have
\(\frac { dy }{ dx }\) = (x¹⁰⁰ + 2x⁵⁰ – 3) \(\frac { d }{ dx }\) (7x⁸ + 20x + 5) + (7x⁸ + 20x + 5) \(\frac { d }{ dx }\) (x¹⁰⁰ + 2x⁵⁰ – 3)
= (x¹⁰⁰ + 2x⁵⁰ – 3) (56x⁷ + 20) + (7x⁸ + 20x + 5) (100x⁹⁹ + 100x⁴⁹)

Question 3.
x (2x – 1)(x + 2)
Solution:
Let y = x (2x – 1) (x + 2); Diff. both sides w.r.t. x, we have
\(\frac { dy }{ dx }\) = x(2x – 1)\(\frac { d }{ dx }\)(x + 2) + x (x + 2) \(\frac { d }{ dx }\)(2x – 1) (x + 2)\(\frac { d }{ dx }\)x
[∵ \(\frac { d }{ dx }\) (uvw) = uw\(\frac { dw }{ dx }\) + uw\(\frac { dv }{ dx }\) + uw\(\frac { du }{ dx }\)]
= x (2x – 1) + 2x (x + 2) + (2x – 1) (x + 2)

Question 4.
(x – 2) (x + 3) (2x + 5)
Solution:
Let y = (x – 2) (x + 3) (2x + 5)
Diff. both sides w.r.t. x; we have
\(\frac { dy }{ dx }\) = (x – 2) (x + 3) \(\frac { d }{ dx }\) (2x + 5) + (x – 2) (2x + 5) \(\frac { d }{ dx }\)(x + 3) + (x + 3) (2x + 5) \(\frac { d }{ dx }\)(x – 2)
= 2 (x – 2) (x + 3) + (x – 2) (2x + 5) + (x + 3) (2x + 5) [using \(\frac { d }{ dx }\) (uvw) = uv\(\frac { dw }{ dx }\) + uw\(\frac { du }{ dx }\) + uw\(\frac { du }{ dx }\)]

Question 5.
y = \(\frac{2 x+5}{3 x-2}\)
Solution:
Let y = \(\frac{2 x+5}{3 x-2}\); Diff. both sides w.r.t. x; we have

Question 6.
y = \(\frac{x^2-3}{x+4}\)
Solution:
Let y = \(\frac{x^2-3}{x+4}\); diff. both sides w.r.t. x; we have

Question 7.
y = \(\frac{2 x-3}{3 x+4}\)
Solution:
Let y = \(\frac{2 x-3}{3 x+4}\); diff. both sides w.r.t. x; we have

Question 8.
y = \(\frac{x^5-x+2}{x^3+7}\)
Solution:
Let y = \(\frac{x^5-x+2}{x^3+7}\); diff. both sides w.r.t. x; we have

Question 9.
s = t² (t + 1)^-1
Solution:
Let s = t² (t + 1)^-1 = \(\frac{t^2}{t+1}\); diff. both sides w.r.t. x; we have

Question 10.
z = \(\frac{u}{u^2+1}\)
Solution:
given z = \(\frac{u}{u^2+1}\); diff. both sides w.r.t. x; we have

Question 11.
y = \(\frac{x^2+2 x+5}{x^3+2 x+4}\)
Solution:
Given y = \(\frac{x^2+2 x+5}{x^3+2 x+4}\); diff. both sides w.r.t. x; we have

Question 12.
f(x) = \(\frac{x^3+2 x}{x^2+4}\)
Solution:
Given f(x) = \(\frac{x^3+2 x}{x^2+4}\); diff. both sides w.r.t. x, we have

Question 13.
If f(x) = \(\frac{x+2}{x-2}\) for all x ≠ 2, find f ‘ ( – 2).
Solution:
Given f(x) = \(\frac{x+2}{x-2}\), x ≠ 2; Diff. both sides w.r.t. x, we have

Question 14.
Differentiate \(\frac{x+2}{x^2-3}\) and find the value of the derivative at x = 0.
Solution:
Given f(x) = \(\frac{x+2}{x^2-3}\); Diff. both sides w.r.t. x, we have

Question 15.
If y = \(\frac{x}{x+a}\), prove that x\(\frac{dy}{dx}\) = y (1 – y).
Solution:
Given y = \(\frac{x}{x+a}\); Diff. both sides w.r.t. x, we have

Question 16.
If \(x \sqrt{1+y}+y \sqrt{1+x}=0\) = 0, prove that \(\frac{d y}{d x}\) = –\(\frac{1}{(1+x)^2}\).
Solution:
Given \(x \sqrt{1+y}+y \sqrt{1+x}=0\) = 0 ⇒ \(x \sqrt{1+y}=-y \sqrt{1+x}\) …(1)
On squaring both sides, we have
x² (1 + y) = y² (1 + x)
⇒ x² + x²y – y² – xy² = 0
⇒(x – y)(x + y) + xy(x – y) = 0
⇒ (x – y) (x + y + xy) = 0
⇒ x + y + xy = 0 [∵ x ≠ y if x = y then given eqn. is meaningless]
⇒ x + y (1 + x) = 0
⇒ y = \(\frac{-x}{1+x}\)
Diff. both sides w.r.t. x, we have

Question 17.
Given that y = \(\sqrt{\frac{1-x}{1+x}}\) show that (1 – x²)\(\frac{dy}{dx}\) + y = 0
Solution:
Given y = \(\sqrt{\frac{1-x}{1+x}}\)
Diff. both sides w.r.t. x, we have

Question 18.
Given that y = (3x – 1)² + (2x – 1)³, find \(\frac{dy}{dx}\) and the points on the curve for which \(\frac{dy}{dx}\) = 0.
Solution:
Given y = (3x – 1)² + (2x – 1)³
Diff. eqn. (1) both sides w.r.t. x, we have
\(\frac{dy}{dx}\) = 2(3x -1)\(\frac{d}{dx}\) (3x -1) + 3 (2x -1)² \(\frac{d}{dx}\) (2x -1)
= 2 (3x – 1) (3.1 -0) + 3(2x – 1)² (2 × 1 – 0) = 6 (3x – 1) + 6 (2x – 1)²
= 6 [3x – 1 + 4x² – 4x + 1] = 6 [4x² – x] = 6x (4x- 1)
Now \(\frac{dy}{dx}\) = 0 ⇒ 6x (4x – 1) = 0 ⇒ x = 0, \(\frac{1}{4}\)
when x = 0 ∴ from (1); y = (0 – 1)² + (0 – 1)³ = 1 – 1 = 0
When x = \(\frac{1}{4}\) ∴ from (1); y = \(\left(\frac{3}{4}-1\right)^2\) + \(\left(\frac{1}{2}-1\right)^3\) = \(\frac{1}{16}\) – \(\frac{1}{8}\) = \(\frac{-1}{16}\)
Hence the required points on curve are (0, 0) and \(\left(\frac{1}{4},-\frac{1}{16}\right) .\)

Question 19.
(i) If y = \(\frac{x-1}{2 x^2-7 x+5}\), find \(\frac{dy}{dx}\) at x = 2.
(ii) If y = \(\frac{x^2+3}{x^3+2 x}\), find \(\frac{dy}{dx}\) at x = 1.
Solution:
(i) Given y = \(\frac{x-1}{2 x^2-7 x+5}\); Diff. both sides w.r.t. x, we have

(ii) Given y = \(\frac{x^2+3}{x^3+2 x}\)
Diff. both sides w.r.t. x, we have

Question 20.
Find the coordinates of the points on the curve y = \(\frac{x}{1-x^2}\) for which \(\frac{dy}{dx}\) = 1.
Solution:
Given eqn. of curve be y = \(\frac{x}{1-x^2}\)
Diff. both sides w.r.t. x, we have

Now \(\frac{dy}{dx}\) = 1
⇒ \(\frac{1+x^2}{\left(1-x^2\right)^2}\) = 1
⇒ 1 + x² = (1 – x²)²
⇒ 1 + x² = 1 + x⁴ – 2x²
⇒ x⁴ – 3x² = 0
⇒ x² (x² – 3) = 0 ⇒ x = 0, ± \(\sqrt{3}\)
When x = 0 ∴ from (1); y = 0
When x = \(\sqrt{3}\)
∴ from (1); y = \(\frac{\sqrt{3}}{1-3}\) = –\(\frac{\sqrt{3}}{2}\)
When x = –\(\sqrt{3}\)
∴ from (1); y = \(\frac{-\sqrt{3}}{1-3}\) = –\(\frac{\sqrt{3}}{2}\)
Hence the coordinates of required points on given curve are (0, 0);
\(\left(\sqrt{3}, \frac{-\sqrt{3}}{2}\right)\) and \(\left(-\sqrt{3}, \frac{\sqrt{3}}{2}\right)\).