Conic Sections Class 11 Maths 1 Exercise 7.2 Solutions Maharashtra Board

Balbharti Maharashtra State Board Class 11 Maths Solutions Pdf Chapter 7 Conic Sections Ex 7.2 Questions and Answers.

11th Maths Part 1 Conic Sections Exercise 7.2 Questions And Answers Maharashtra Board

Question 1.
Find the
(i) lengths of the principal axes
(ii) co-ordinates of the foci
(iii) equations of directrices
(iv) length of the latus rectum
(v) distance between foci
(vi) distance between directrices of the ellipse:
(a) $\frac{x^{2}}{25}+\frac{y^{2}}{9}=1$
(b) 3x² + 4y² = 12
(c) 2x² + 6y² = 6
(d) 3x² + 4y² = 1
Solution:
(a) Given equation of the ellipse is $\frac{x^{2}}{25}+\frac{y^{2}}{9}=1$
Comparing this equation with $\frac{x^{2}}{\mathrm{a}^{2}}+\frac{y^{2}}{\mathrm{~b}^{2}}=1$, we get
a² = 25 and b² = 9
a = 5 and b = 3
Since a > b,
X-axis is the major axis and Y-axis is the minor axis.

(i) Length of major axis = 2a = 2(5) = 10
Length of minor axis = 2b = 2(3) = 6
Lengths of the principal axes are 10 and 6.

(ii) We know that e = $\frac{\sqrt{a^{2}-b^{2}}}{a}$
= $\frac{\sqrt{25-9}}{5}$
= $\frac{\sqrt{16}}{5}$
= $\frac{4}{5}$
Co-ordinates of the foci are S(ae, 0) and S'(-ae, 0),
i.e., S(5($\frac{4}{5}$), 0) and S'(-5($\frac{4}{5}$), 0)
i.e., S(4, 0) and S'(-4, 0)

(iii) Equations of the directrices are x = ±$\frac{\mathrm{a}}{\mathrm{e}}$
= $\pm \frac{5}{\frac{4}{5}}$
= $\pm \frac{25}{4}$

(iv) Length of latus rectum = $\frac{2 b^{2}}{a}=\frac{2(3)^{2}}{5}=\frac{18}{5}$

(v) Distance between foci = 2ae
= 2(5)($\frac{4}{5}$)
= 8

(vi) Distance between directrices = $\frac{2 \mathrm{a}}{\mathrm{e}}$
= $\frac{2(5)}{\frac{4}{5}}$
= $\frac{25}{2}$

(b) Given equation of the ellipse is 3x² + 4y² = 12
$\frac{x^{2}}{4}+\frac{y^{2}}{3}=1$
Comparing this equation with $\frac{x^{2}}{\mathrm{a}^{2}}+\frac{y^{2}}{\mathrm{~b}^{2}}=1$, we get
a² = 4 and b² = 3
a = 2 and b = √3
Since a > b,
X-axis is the major axis and Y-axis is the minor axis.

(i) Length of major axis = 2a = 2(2) = 4
Length of minor axis = 2b = 2√3
Lengths of the principal axes are 4 and 2√3.

(ii) We know that e = $\frac{\sqrt{a^{2}-b^{2}}}{a}$
= $\frac{\sqrt{4-3}}{2}$
= $\frac{1}{2}$
Co-ordinates of the foci are S(ae, 0) and S'(-ae, 0),
i.e., S(2($\frac{1}{2}$), 0) and S'(-2($\frac{1}{2}$), 0)
i.e., S(1, 0) and S'(-1, 0)

(iii) Equations of the directrices are x = ±$\frac{\mathrm{a}}{\mathrm{e}}$
= $\pm \frac{2}{\frac{1}{2}}$
= ±4

(iv) Length of latus rectum = $\frac{2 b^{2}}{a}=\frac{2(\sqrt{3})^{2}}{2}=3$

(v) Distance between foci = 2ae = 2(2)($\frac{1}{2}$) = 2

(vi) Distance between directrices = $\frac{2 \mathrm{a}}{\mathrm{e}}$
= $\frac{2(2)}{\frac{1}{2}}$
= 8

(c) Given equation of the ellipse is 2x² + 6y² = 6
$\frac{x^{2}}{3}+\frac{y^{2}}{1}=1$
Comparing this equation with $\frac{x^{2}}{\mathrm{a}^{2}}+\frac{y^{2}}{\mathrm{~b}^{2}}=1$, we get
a² = 3 and b² = 1
a = √3 and b = 1
Since a > b,
X-axis is the major axis and Y-axis is the minor axis.

(i) Length of major axis = 2a = 2√3
Length of minor axis = 2b = 2(1) = 2
Lengths of the principal axes are 2√3 and 2.

(ii) We know that e = $\frac{\sqrt{a^{2}-b^{2}}}{a}$
= $\frac{\sqrt{3-1}}{\sqrt{3}}$
= $\frac{\sqrt{2}}{\sqrt{3}}$
Co-ordinates of the foci are S(ae, 0) and S'(-ae, 0),
i.e., S(√3($\frac{\sqrt{2}}{\sqrt{3}}$), o) and S'(-√3($\frac{\sqrt{2}}{\sqrt{3}}$), 0)
i.e., S(√2, 0) and S'(-√2, 0)

(iii) Equations of the directrices are x = ±$\frac{a}{e}$,
= $\pm \frac{\sqrt{3}}{\frac{\sqrt{2}}{\sqrt{3}}}$
= $\pm \frac{3}{\sqrt{2}}$

(iv) Length of latus rectum = $\frac{2 b^{2}}{a}=\frac{2(1)^{2}}{\sqrt{3}}=\frac{2}{\sqrt{3}}$

(v) Distance between foci = 2ae
= $2(\sqrt{3})\left(\frac{\sqrt{2}}{\sqrt{3}}\right)$
= 2√2

(vi) Distance between directrices = $\frac{2 \mathrm{a}}{\mathrm{e}}$
= $\frac{2 \sqrt{3}}{\frac{\sqrt{2}}{\sqrt{3}}}$
= $\frac{2 \times 3}{\sqrt{2}}$
= 3√2

(d) Given equation of the ellipse is 3x² + 4y = 1.
$\frac{x^{2}}{\frac{1}{3}}+\frac{y^{2}}{\frac{1}{4}}=1$
Comparing this equation with $\frac{x^{2}}{\mathrm{a}^{2}}+\frac{y^{2}}{\mathrm{~b}^{2}}=1$, we get
a² = $\frac{1}{3}$ and b² = $\frac{1}{4}$
a = $\frac{1}{\sqrt{3}}$ and b = $\frac{1}{2}$
Since a > b,
X-axis is the major axis and Y-axis is the minor axis.

(i) Length of major axis = 2a = 2($\frac{1}{\sqrt{3}}$) = $\frac{2}{\sqrt{3}}$
Length of minor axis = 2b = 2($\frac{1}{2}$) = 1
Lengths of the principal axes are $\frac{2}{\sqrt{3}}$ and 1.

(ii) We know that e = $\frac{\sqrt{a^{2}-b^{2}}}{a}$
e = $\frac{\sqrt{\frac{1}{3}-\frac{1}{4}}}{\frac{1}{\sqrt{3}}}=\frac{\sqrt{\frac{1}{12}}}{\frac{1}{\sqrt{3}}}=\sqrt{\frac{3}{12}}=\sqrt{\frac{1}{4}}=\frac{1}{2}$
Co-ordinates of the foci are S(ae, 0) and S'(-ae, 0),
i.e., S$\left(\frac{1}{\sqrt{3}}\left(\frac{1}{2}\right), 0\right)$ and S’$\left(-\frac{1}{\sqrt{3}}\left(\frac{1}{2}\right), 0\right)$
i.e., S($\frac{1}{2 \sqrt{3}}$, 0) and S'(-$\frac{1}{2 \sqrt{3}}$, 0)

(iii) Equations of the directrices are x = ±$\frac{a}{e}$,
= $\pm \frac{\frac{1}{\sqrt{3}}}{\frac{1}{2}}$
= $\pm \frac{2}{\sqrt{3}}$

(iv) Length of latus rectum = $\frac{2 b^{2}}{a}$
= $\frac{2\left(\frac{1}{2}\right)^{2}}{\frac{1}{\sqrt{3}}}$
= $\frac{\sqrt{3}}{2}$

(v) Distance between foci = 2ae
= $2\left(\frac{1}{\sqrt{3}}\right)\left(\frac{1}{2}\right)$
= $\frac{1}{\sqrt{3}}$

(vi) Distance between directrices = $\frac{2 a}{e}$
= $\frac{2\left(\frac{1}{\sqrt{3}}\right)}{\frac{1}{2}}$
= $\frac{4}{\sqrt{3}}$

Question 2.
Find the equation of the ellipse in standard form if
(i) eccentricity = $\frac{3}{8}$ and distance between its foci = 6.
(ii) the length of the major axis is 10 and the distance between foci is 8.
(iii) distance between directrices is 18 and eccentricity is $\frac{1}{3}$.
(iv) minor axis is 16 and eccentricity is $\frac{1}{3}$.
(v) the distance between foci is 6 and the distance between directrices is $\frac{50}{3}$.
(vi) the latus rectum has length 6 and foci are (±2, 0).
(vii) passing through the points (-3, 1) and (2, -2).
(viii) the distance between its directrices is 10 and which passes through (-√5, 2).
(ix) eccentricity is $\frac{2}{3}$ and passes through (2, $\frac{-5}{3}$).
Solution:
(i) Let the required equation of ellipse be $\frac{x^{2}}{a^{2}}+\frac{y^{2}}{b^{2}}=1$, where a > b.
Given, eccentricity (e) = $\frac{3}{8}$
Distance between foci = 2ae
Given, distance between foci = 6
2ae = 6

The required equation of ellipse is $\frac{x^{2}}{64}+\frac{y^{2}}{55}=1$.

(ii) Let the required equation of ellipse be $\frac{x^{2}}{a^{2}}+\frac{y^{2}}{b^{2}}=1$, where a > b.
Length of major axis = 2a
Given, length of major axis = 10
2a = 10
a = 5
a² = 25
Distance between foci = 2ae
Given, distance between foci = 8
2ae = 8
2(5)e = 8

The required equation of ellipse is $\frac{x^{2}}{25}+\frac{y^{2}}{9}=1$.

(iii) Let the required equation of ellipse be $\frac{x^{2}}{a^{2}}+\frac{y^{2}}{b^{2}}=1$, where a > b.
Given, eccentricity (e) = $\frac{1}{3}$
Distance between directrices = $\frac{2a}{e}$
Given, distance between directrices = 18

The required equation of ellipse is $\frac{x^{2}}{9}+\frac{y^{2}}{8}=1$

(iv) Let the required equation of ellipse be $\frac{x^{2}}{a^{2}}+\frac{y^{2}}{b^{2}}=1$, where a > b.
Length of minor axis = 2b
Given, length of minor axis = 16
2b = 16
b = 8
b² = 64
Given, eccentricity (e) = $\frac{1}{3}$
Now, b² = a² (1 – e²)

The required equation of ellipse is $\frac{x^{2}}{72}+\frac{y^{2}}{64}=1$.

(v) Let the required equation of ellipse be $\frac{x^{2}}{a^{2}}+\frac{y^{2}}{b^{2}}=1$, where a > b.
Distance between foci = 2ae
Given, distance between foci = 6
2ae = 6
ae = 3
a = $\frac{3}{e}$ …….(i)
Distance between directrices = $\frac{2a}{e}$
Given, distance between directrices = $\frac{50}{3}$

The required equation of ellipse is $\frac{x^{2}}{25}+\frac{y^{2}}{16}=1$.

(vi) Given, the length of the latus rectum is 6, and co-ordinates of foci are (±2, 0).
The foci of the ellipse are on the X-axis.
Let the required equation of ellipse be $\frac{x^{2}}{a^{2}}+\frac{y^{2}}{b^{2}}=1$, where a > b.
Length of latus rectum = $\frac{2 b^{2}}{a}$
$\frac{2 b^{2}}{a}$ = 6
b² = 3a …..(i)
Co-ordinates of foci are (±ae, 0)
ae = 2
a²e² = 4 …..(ii)
Now, b² = a² (1 – e²)
b² = a² – a² e²
3a = a² – 4 …..[From (i) and (ii)]
a² – 3a – 4 = 0
a² – 4a + a – 4 = 0
a(a – 4) + 1(a – 4) = 0
(a – 4) (a + 1) = 0
a – 4 = 0 or a + 1 = 0
a = 4 or a = -1
Since a = -1 is not possible,
a = 4
a² = 16
Substituting a = 4 in (i), we get
b² = 3(4) = 12
The required equation of ellipse is $\frac{x^{2}}{16}+\frac{y^{2}}{12}=1$.

(vii) Let the required equation of ellipse be $\frac{x^{2}}{a^{2}}+\frac{y^{2}}{b^{2}}=1$, where a > b.
The ellipse passes through the points (-3, 1) and (2, -2).
Substituting x = -3 and y = 1 in equation of ellipse, we get

Equations (i) and (ii) become
9A + B = 1 …..(iii)
4A + 4B = 1 …..(iv)
Multiplying (iii) by 4, we get
36A + 4B = 4 …..(v)
Subtracting (iv) from (v), we get
32A = 3
A = $\frac{3}{32}$
Substituting A = $\frac{3}{32}$ in (iv), we get
4($\frac{3}{32}$) + 4B = 1
$\frac{3}{8}$ + 4B = 1
4B = 1 – $\frac{3}{8}$
4B = $\frac{5}{8}$
B = $\frac{5}{32}$

(viii) Let the required equation of ellipse be $\frac{x^{2}}{a^{2}}+\frac{y^{2}}{b^{2}}=1$, where a > b.
Distance between directrices = $\frac{2 a}{e}$
Given, distance between directrices = 10
$\frac{2 a}{e}$ = 10
a = 5e …..(i)
The ellipse passes through (-√5, 2).
Substituting x = -√5 and y = 2 in equation of ellipse, we get

b² = 15($\frac{2}{5}$)
b² = 6
The required equation of ellipse is $\frac{x^{2}}{15}+\frac{y^{2}}{6}=1$.

(ix) Let the required equation of ellipse be $\frac{x^{2}}{a^{2}}+\frac{y^{2}}{b^{2}}=1$, where a > b.
Given, eccentricity (e) = $\frac{2}{3}$
The ellipse passes through (2, $\frac{-5}{3}$).
Substituting x = 2 and y = $\frac{-5}{3}$ in equation of ellipse, we get

The required equation of ellipse is $\frac{x^{2}}{9}+\frac{y^{2}}{5}=1$.

Question 3.
Find the eccentricity of an ellipse, if the length of its latus rectum is one-third of its minor axis.
Solution:
Let the equation of ellipse be $\frac{x^{2}}{\mathrm{a}^{2}}+\frac{y^{2}}{\mathrm{~b}^{2}}=1$, where a > b.
Length of latus rectum = $\frac{2 b^{2}}{a}$
Length of minor axis = 2b
According to the given condition,
Length of latus rectum = $\frac{1}{3}$ (Minor axis)

Question 4.
Find the eccentricity of an ellipse, if the distance between its directrices is three times the distance between its foci.
Solution:
Let the required equation of ellipse be $\frac{x^{2}}{\mathrm{a}^{2}}+\frac{y^{2}}{\mathrm{~b}^{2}}=1$, where a > b.
Distance between directrices = $\frac{2 \mathrm{a}}{\mathrm{e}}$
Distance between foci = 2ae
According to the given condition,
distance between directrices = 3(distance between foci)
$\frac{2 \mathrm{a}}{\mathrm{e}}$ = 3(2ae)
$\frac{1}{\mathrm{e}}$ = 3e
$\frac{1}{3}$ = e²
e = $\frac{1}{\sqrt{3}}$ ……[∵ 0 < e < 1]
Eccentricity of the ellipse is $\frac{1}{\sqrt{3}}$

Question 5.
Show that the product of the lengths of the perpendicular segments drawn from the foci to any tangent line to the ellipse $\frac{x^{2}}{25}+\frac{y^{2}}{16}=1$ is equal to 16.
Solution:
Given equation of the ellipse is $\frac{x^{2}}{25}+\frac{y^{2}}{16}=1$.
Comparing this equation with $\frac{x^{2}}{\mathrm{a}^{2}}+\frac{y^{2}}{\mathrm{~b}^{2}}=1$, we get
a² = 25, b² = 16
a = 5, b = 4

Question 6.
A tangent having slope $\left(-\frac{1}{2}\right)$ the ellipse 3x² + 4y² = 12 intersects the X and Y axes in the points A and B respectively. If O is the origin, find the area of the triangle AOB.
Solution:
Given equation of the ellipse is 3x² + 4y² = 12.
$\frac{x^{2}}{4}+\frac{y^{2}}{3}=1$
Comparing this equation with $\frac{x^{2}}{a^{2}}+\frac{y^{2}}{b^{2}}=1$, we get
a² = 4, b² = 3
Equations of tangents to the ellipse $\frac{x^{2}}{a^{2}}+\frac{y^{2}}{b^{2}}=1$ having slope m are
y = mx ± $\sqrt{a^{2} m^{2}+b^{2}}$
Here, m = $-\frac{1}{2}$
Equations of the tangents are
y = $\frac{-1}{2} x \pm \sqrt{4\left(-\frac{1}{2}\right)^{2}+3}=\frac{-1}{2} x \pm 2$
2y = -x ± 4
x + 2y ± 4 = 0
Consider the tangent x + 2y – 4 = 0
Let this tangent intersect the X-axis at A(x₁, 0) and Y-axis at B(0, y₁).
x₁ + 0 – 4 = 0 and 0 + 2y₁ – 4 = 0
x₁ = 4 and y₁ = 2
A = (4, 0) and B = (0, 2)
l(OA) = 4 and l(OB) = 2

Area of ∆AOB = $\frac{1}{2}$ × l(OA) × l(OB)
= $\frac{1}{2}$ × 4 × 2
= 4 sq.units

Question 7.
Show that the line x – y = 5 is a tangent to the ellipse 9x² + 16y² = 144. Find the point of contact.
Solution:
Given equation of the ellipse is 9x² + 16y² = 144
$\frac{x^{2}}{16}+\frac{y^{2}}{9}=1$
Comparing this equation with $\frac{x^{2}}{a^{2}}+\frac{y^{2}}{b^{2}}=1$, we get
a² = 16 and b² = 9
Given equation of line is x – y = 5, i.e., y = x – 5
c² = a² m² + b²
Comparing this equation with y = mx + c, we get
m = 1 and c = -5
For the line y = mx + c to be a tangent to the ellipse $\frac{x^{2}}{a^{2}}+\frac{y^{2}}{b^{2}}=$1, we must have
c² = a² m² + b²
c² = (-5)² = 25
a² m² + b² = 16(1)² + 9 = 16 + 9 = 25 = c²
The given line is a tangent to the given ellipse and point of contact
= $\left(\frac{-\mathrm{a}^{2} \mathrm{~m}}{\mathrm{c}}, \frac{\mathrm{b}^{2}}{\mathrm{c}}\right)$
= $\left(\frac{(-16)(1)}{-5}, \frac{9}{-5}\right)$
= $\left(\frac{16}{5}, \frac{-9}{5}\right)$

Question 8.
Show that the line 8y + x = 17 touches the ellipse x² + 4y² = 17. Find the point of contact.
Solution:
Given equation of the ellipse is x² + 4y² = 17.
$\frac{x^{2}}{17}+\frac{y^{2}}{\frac{17}{4}}=1$
Comparing this equation with $\frac{x^{2}}{\mathrm{a}^{2}}+\frac{y^{2}}{\mathrm{~b}^{2}}=1$, we get
a² = 17 and b² = $\frac{17}{4}$
Given equation of line is 8y + x = 17,
y = $\frac{-1}{8} x+\frac{17}{8}$
Comparing this equation with y = mx + c, we get
m = $\frac{-1}{8}$ and c = $\frac{17}{8}$
For the line y = mx + c to be a tangent to the ellipse $\frac{x^{2}}{a^{2}}+\frac{y^{2}}{b^{2}}=$1, we must have

Question 9.
Determine whether the line x + 3y√2 = 9 is a tangent to the ellipse $\frac{x^{2}}{9}+\frac{y^{2}}{4}=1$. If so, find the co-ordinates of the point of contact.
Solution:
Given equation of the ellipse is $\frac{x^{2}}{9}+\frac{y^{2}}{4}=1$
Comparing this equation with $\frac{x^{2}}{\mathrm{a}^{2}}+\frac{y^{2}}{\mathrm{~b}^{2}}=1$, we get
a² = 9 and b² = 4
Given equation of line is x + 3y√2 = 9,
i.e., y = $\frac{-1}{3 \sqrt{2}} x+\frac{3}{\sqrt{2}}$
Comparing this equation with y = mx + c, we get
m = $\frac{-1}{3 \sqrt{2}}$ and c = $\frac{3}{\sqrt{2}}$
For the line y = mx + c to be a tangent to the ellipse $\frac{x^{2}}{\mathrm{a}^{2}}+\frac{y^{2}}{\mathrm{~b}^{2}}=$1, we must have

Question 10.
Find k, if the line 3x + 4y + k = 0 touches 9x² + 16y² = 144.
Solution:
Given equation of the ellipse is 9x² + 16y² = 144.
$\frac{x^{2}}{16}+\frac{y^{2}}{9}=1$
Comparing this equation with $\frac{x^{2}}{\mathrm{a}^{2}}+\frac{y^{2}}{\mathrm{~b}^{2}}=1$, we get
a² = 16 and b² = 9
Given equation of line is 3x + 4y + k = 0,
i.e., y = $-\frac{3}{4} x-\frac{k}{4}$
Comparing this equation with y = mx + c, we get
m = $\frac{-3}{4}$ and c = $\frac{-\mathrm{k}}{4}$
For the line y = mx + c to be a tangent to the ellipse $\frac{x^{2}}{a^{2}}+\frac{y^{2}}{b^{2}}=$1, we must have
c² = a² m² + b²
$\left(\frac{-k}{4}\right)^{2}=16\left(\frac{-3}{4}\right)^{2}+9$
$\frac{\mathrm{k}^{2}}{16}$ = 9 + 9
$\frac{\mathrm{k}^{2}}{16}$ = 18
k² = 288
k = ±12√2

Question 11.
Find the equations of the tangents to the ellipse:
(i) $\frac{x^{2}}{5}+\frac{y^{2}}{4}=1$ passing through the point (2, -2).
(ii) 4x² + 7y² = 28 from the point (3, -2).
(iii) 2x² + y² = 6 from the point (2, 1).
(iv) x² + 4y² = 9 which are parallel to the line 2x + 3y – 5 = 0.
(v) $\frac{x^{2}}{25}+\frac{y^{2}}{4}=1$ which are parallel to the line x + y + 1 = 0.
(vi) 5x² + 9y² = 45 which are ⊥ to the line 3x + 2y + 1 = 0.
(vii) x² + 4y² = 20 which are ⊥ to the line 4x + 3y = 7.
Solution:
(i) Given equation of the ellipse is $\frac{x^{2}}{5}+\frac{y^{2}}{4}=1$.
Comparing this equation with $\frac{x^{2}}{\mathrm{a}^{2}}+\frac{y^{2}}{\mathrm{~b}^{2}}=1$, we get
a² = 5 and b² = 4
Equations of tangents to the ellipse $\frac{x^{2}}{\mathrm{a}^{2}}+\frac{y^{2}}{\mathrm{~b}^{2}}=1$ having slope m are
y = mx ± $\sqrt{a^{2} m^{2}+b^{2}}$
Since (2, -2) lies on both the tangents,
-2 = 2m ±$\sqrt{5 m^{2}+4}$
-2 – 2m = ±$\sqrt{5 m^{2}+4}$
Squaring both the sides, we get
4m² + 8m + 4 = 5m² + 4
m² – 8m = 0
m(m – 8) = 0
m = 0 or m = 8
These are the slopes of the required tangents.
By slope point form y – y₁ = m(x – x₁),
the equations of the tangents are
y + 2 = 0(x – 2) and y + 2 = 8(x – 2)
y + 2 = 0 and y + 2 = 8x – 16
y + 2 = 0 and 8x – y – 18 = 0

(ii) Given equation of the ellipse is 4x² + 7y² = 28.
$\frac{x^{2}}{7}+\frac{y^{2}}{4}=1$
Comparing this equation with $\frac{x^{2}}{\mathrm{a}^{2}}+\frac{y^{2}}{\mathrm{~b}^{2}}=1$, we get
a² = 7 and b² = 4
Equations of tangents to the ellipse $\frac{x^{2}}{\mathrm{a}^{2}}+\frac{y^{2}}{\mathrm{~b}^{2}}=1$ having slope m are
y = mx ± $\sqrt{a^{2} m^{2}+b^{2}}$
Since (3, -2) lies on both the tangents,
-2 = 3m ± $\sqrt{7 \mathrm{~m}^{2}+4}$
-2 – 3m = ±$\sqrt{7 \mathrm{~m}^{2}+4}$
Squaring both the sides, we get
9m² + 12m + 4 = 7m² + 4
2m² + 12m = 0
2m(m + 6) = 0
m = 0 or m = -6
These are the slopes of the required tangents.
By slope point form y – y₁ = m(x – x₁),
the equations of the tangents are
y + 2 = 0(x – 3) and y + 2 = -6(x – 3)
y + 2 = 0 and y + 2 = -6x + 18
y + 2 = 0 and 6x + y – 16 = 0

(iii) Given equation of the ellipse is 2x² + y² = 6.
$\frac{x^{2}}{3}+\frac{y^{2}}{6}=1$
Comparing this equation with $\frac{x^{2}}{a^{2}}+\frac{y^{2}}{b^{2}}=1$, we get
a² = 3 and b² = 6
Equations of tangents to the ellipse $\frac{x^{2}}{a^{2}}+\frac{y^{2}}{b^{2}}=1$ having slope m are
y = mx ± $\sqrt{a^{2} m^{2}+b^{2}}$
Since (2, 1) lies on both the tangents,
1 = 2m ± $\sqrt{3 m^{2}+6}$
1 – 2m = ±$\sqrt{3 m^{2}+6}$
Squaring both the sides, we get
1 – 4m + 4m² = 3m² + 6
m² – 4m – 5 = 0
(m – 5)(m + 1) = 0
m = 5 or m = -1
These are the slopes of the required tangents.
By slope point form y – y₁ = m(x – x₁),
the equations of the tangents are
y – 1 = 5(x – 2) and y – 1 = -1(x – 2)
y – 1 = 5x – 10 and y – 1 = -x + 2
5x – y – 9 = 0 and x + y – 3 = 0

(iv) Given equation of the ellipse is x² + 4y² = 9.
$\frac{x^{2}}{9}+\frac{y^{2}}{\frac{9}{4}}=1$
Comparing this equation with $\frac{x^{2}}{a^{2}}+\frac{y^{2}}{b^{2}}=1$, we get
a² = 9 and b² = $\frac{9}{4}$
Slope of the line 2x + 3y – 5 = 0 is $\frac{-2}{3}$.
Since the given line is parallel to the required tangents, slope of the required tangents is
m = $\frac{-2}{3}$
Equations of tangents to the ellipse $\frac{x^{2}}{a^{2}}+\frac{y^{2}}{b^{2}}=1$ having slope m are

(v) Given equation of the ellipse is $\frac{x^{2}}{25}+\frac{y^{2}}{4}=1$.
Comparing this equation with $\frac{x^{2}}{a^{2}}+\frac{y^{2}}{b^{2}}=1$, we get
a² = 25 and b² = 4
Slope of the given line x + y + 1 = 0 is -1.
Since the given line is parallel to the required tangents,
the slope of the required tangents is m = -1.
Equations of tangents to the ellipse

(vi) Given equation of the ellipse is 5x² + 9y² = 45.
$\frac{x^{2}}{9}+\frac{y^{2}}{5}=1$
Comparing this equation with $\frac{x^{2}}{a^{2}}+\frac{y^{2}}{b^{2}}=1$, we get
a² = 9 and b² = 5
Slope of the given line 3x + 2y + 1 = 0 is $\frac{-3}{2}$
Since the given line is perpendicular to the required tangents, slope of the required tangents is
m = $\frac{2}{3}$
Equations of tangents to the ellipse

(vii) Given equation of the ellipse is x² + 4y² = 20.
$\frac{x^{2}}{20}+\frac{y^{2}}{5}=1$
Comparing this equation with $\frac{x^{2}}{\mathrm{a}^{2}}+\frac{y^{2}}{\mathrm{~b}^{2}}=1$, we get
a² = 20 and b² = 5
Slope of the given line 4x + 3y = 7 is $\frac{-4}{3}$.
Since the given line is perpendicular to the required tangents,
slope of the required tangents is m = $\frac{3}{4}$.
Equations of tangents to the ellipse $\frac{x^{2}}{\mathrm{a}^{2}}+\frac{y^{2}}{\mathrm{~b}^{2}}=1$ having slope m are
y = mx ± $\sqrt{a^{2} m^{2}+b^{2}}$
y = $\frac{3}{4} x \pm \sqrt{20\left(\frac{3}{4}\right)^{2}+5}$

Question 12.
Find the equation of the locus of a point, the tangents from which to the ellipse 3x² + 5y² = 15 are at right angles.
Solution:
Given equation of the ellipse is 3x² + 5y² = 15.
$\frac{x^{2}}{5}+\frac{y^{2}}{3}=1$
Comparing this equation with $\frac{x^{2}}{\mathrm{a}^{2}}+\frac{y^{2}}{\mathrm{~b}^{2}}=1$, we get
a² = 5 and b² = 3
Equations of tangents to the ellipse $\frac{x^{2}}{\mathrm{a}^{2}}+\frac{y^{2}}{\mathrm{~b}^{2}}=1$ having slope m are
y = mx ± $\sqrt{a^{2} m^{2}+b^{2}}$
y = mx ± $\sqrt{5 m^{2}+3}$
y – mx =±$\sqrt{5 m^{2}+3}$
Squaring both the sides, we get
y² – 2mxy + m²x² = 5m² + 3
(x² – 5) m² – 2xym + (y² – 3) = 0
The roots m₁ and m₂ of this quadratic equation are the slopes of the tangents.
m₁m₂ = $\frac{y^{2}-3}{x^{2}-5}$
Since the tangents are at right angles,
m₁m₂ = -1
$\frac{y^{2}-3}{x^{2}-5}=-1$
y² – 3 = -x² + 5
x² + y² = 8, which is the required equation of the locus.

Alternate method:
The locus of the point of intersection of perpendicular tangents is the director circle of an ellipse.
The equation of the director circle of an ellipse $\frac{x^{2}}{a^{2}}+\frac{y^{2}}{b^{2}}=1$ is x² + y² = a² + b²
Here, a² = 5 and b² = 3
x² + y² = 5 + 3
x² + y² = 8, which is the required equation of the locus.

Question 13.
Tangents are drawn through a point P to the ellipse 4x² + 5y² = 20 having inclinations θ₁ and θ₂ such that tan θ₁ + tan θ₂ = 2. Find the equation of the locus of P.
Solution:
Given equation of the ellipse is 4x² + 5y² = 20.
$\frac{x^{2}}{5}+\frac{y^{2}}{4}=1$
Comparing this equation with $\frac{x^{2}}{a^{2}}+\frac{y^{2}}{b^{2}}=1$, we get
a² = 5 and b² = 4
Since inclinations of tangents are θ₁ and θ₂,
m₁ = tan θ₁ and m₂ = tan θ₂
Equation of tangents to the ellipse $\frac{x^{2}}{a^{2}}+\frac{y^{2}}{b^{2}}=1$ having slope m are
y = mx ± $\sqrt{a^{2} m^{2}+b^{2}}$
y = mx ± $\sqrt{5 \mathrm{~m}^{2}+4}$
y – mx = ± $\sqrt{5 \mathrm{~m}^{2}+4}$
Squaring both the sides, we get
y² – 2mxy + m²x² = 5m² + 4
(x² – 5)m² – 2xym + (y² – 4) = 0
The roots m₁ and m₂ of this quadratic equation are the slopes of the tangents.
m₁ + m₂ = $\frac{-(-2 x y)}{x^{2}-5}=\frac{2 x y}{x^{2}-5}$
Given, tan θ₁ + tan θ₂ = 2
m₁ + m₂ = 2
$\frac{2 x y}{x^{2}-5}$ = 2
xy = x² – 5
x² – xy – 5 = 0, which is the required equation of the locus of P.

Question 14.
Show that the locus of the point of intersection of tangents at two points on an ellipse, whose eccentric angles differ by a constant, is an ellipse.
Solution:
Let P(θ₁) and Q(θ₂) be any two points on the given ellipse such that θ₁ – θ₂ = k, where k is a constant.
The equation of the tangent at point P(θ₁) is
$\frac{x \cos \theta_{1}}{\mathrm{a}}+\frac{y \sin \theta_{1}}{\mathrm{~b}}=1$ ……(i)
The equation of the tangent at point Q(θ₂) is
$\frac{x \cos \theta_{2}}{\mathrm{a}}+\frac{y \sin \theta_{2}}{\mathrm{~b}}=1$ ……(ii)
Multiplying equation (i) by cos θ₂ and equation (ii) by cos θ₁ and subtracting, we get
$\frac{y}{b}$ (sin θ₁ cos θ₂ – sin θ₂ cos θ₁) = cos θ₂ – cos θ₁

Question 15.
P and Q are two points on the ellipse $\frac{x^{2}}{a^{2}}+\frac{y^{2}}{b^{2}}=1$ with eccentric angles θ₁ and θ₂. Find the equation of the locus of the point of intersection of the tangents at P and Q if θ₁ + θ₂ = $\frac{\pi}{2}$.
Solution:
Given equation of the ellipse is $\frac{x^{2}}{a^{2}}+\frac{y^{2}}{b^{2}}=1$.
θ₁ and θ₂ are the eccentric angles of a tangent.
Equation of tangent at point P is
$\frac{x}{a} \cos \theta_{1}+\frac{y}{b} \sin \theta_{1}=1$ ……(i)
Equation of tangent at point Q is
$\frac{x}{a} \cos \theta_{2}+\frac{y}{b} \sin \theta_{2}=1$ ………(ii)
θ₁ + θ₂ = $\frac{\pi}{2}$ …..[Given]

i.e., points P and Q coincide, which is not possible, as P and Q are two different points.
cos θ₁ – sin θ₁ ≠ 0
Dividing equation (iii) by (cos θ₁ – sin θ₁), we get
$\frac{x_{1}}{a}=\frac{y_{1}}{b}$
bx₁ – ay₁ = 0
bx – ay = 0, which is the required equation of locus of point M.

Question 16.
The eccentric angles of two points P and Q of the ellipse 4x² + y² = 4 differ by $\frac{2 \pi}{3}$. Show that the locus of the point of intersection of the tangents at P and Q is the ellipse 4x² + y² = 16.
Solution:
Given equation of the ellipse is 4x² + y² = 4.
$\frac{x^{2}}{1}+\frac{y^{2}}{4}=1$
Let P(θ₁) and Q(θ₂) be any two points on the given ellipse such that
θ₁ – θ₂ = $\frac{2 \pi}{3}$
The equation of the tangent at point P(θ₁) is
$\frac{x \cos \theta_{1}}{1}+\frac{y \sin \theta_{1}}{2}=1$ ……(i)
The equation of the tangent at point Q(θ₂) is
$\frac{x \cos \theta_{2}}{1}+\frac{y \sin \theta_{2}}{2}=1$
Multiplying equation (i) by cos θ₂ and equation (ii) by cos θ₁ and subtracting, we get

Question 17.
Find the equations of the tangents to the ellipse $\frac{x^{2}}{16}+\frac{y^{2}}{9}=1$, making equal intercepts on co-ordinate axes.
Solution:
Given equation of the ellipse is $\frac{x^{2}}{16}+\frac{y^{2}}{9}=1$
Comparing this equation with $\frac{x^{2}}{a^{2}}+\frac{y^{2}}{b^{2}}=1$, we get
a² = 16 and b² = 9
Since the tangents make equal intercepts on the co-ordinate axes,
m = -1.
Equations of tangents to the ellipse $\frac{x^{2}}{a^{2}}+\frac{y^{2}}{b^{2}}=1$ having slope m are
y = mx ± $\sqrt{a^{2} m^{2}+b^{2}}$
y = -x ± $\sqrt{16(-1)^{2}+9}$
y = -x ± $\sqrt{25}$
x + y = ±5

Question 18.
A tangent having slope $\left(-\frac{1}{2}\right)$ to the ellipse 3x² + 4y² = 12 intersects the X and Y axes in the points A and B respectively. If O is the origin, find the area of the triangle AOB.
Solution:
The equation of the ellipse is 3x² + 4y² = 12
$\frac{x^{2}}{4}+\frac{y^{2}}{3}=1$
Comparing with $\frac{x^{2}}{a^{2}}+\frac{y^{2}}{b^{2}}=1$, we get
a² = 4, b² = 3
The equation of tangent with slope m is

It meets X axis at A
∴ for A, put y = 0 in equation (1), we get,
x = ±4
∴ A = (±4, 0)
Similarly, B = (0, ±2)
∴ OA = 4, OB = 2
∴ Area of ΔOAB = $\frac{1}{2}$ × OA × OB
= $\frac{1}{2}$ × 4 × 2
= 4 sq. units

Class 11 Maharashtra State Board Maths Solution 

• Exercise 6.1 Class 11 Maths 1
• Exercise 6.2 Class 11 Maths 1
• Exercise 6.3 Class 11 Maths 1
• Miscellaneous Exercise 6 Class 11 Maths 1
• Exercise 7.1 Class 11 Maths 1
• Exercise 7.2 Class 11 Maths 1
• Exercise 7.3 Class 11 Maths 1
• Miscellaneous Exercise 7 Class 11 Maths 1
• Exercise 8.1 Class 11 Maths 1
• Exercise 8.2 Class 11 Maths 1
• Exercise 8.3 Class 11 Maths 1
• Miscellaneous Exercise 8 Class 11 Maths 1