Gujarati
10-2. Parabola, Ellipse, Hyperbola
easy

The distance between the directrices of the ellipse $\frac{{{x^2}}}{{36}} + \frac{{{y^2}}}{{20}} = 1$ is

A

$8$

B

$12$

C

$18$

D

$24$

Solution

(c) $a = 6,\;b = 2\sqrt 5 $

${b^2} = {a^2}(1 – {e^2})$ 

$\frac{{20}}{{36}} = (1 – {e^2})$

$e = \sqrt {\frac{{16}}{{36}}} = \frac{2}{3}$

But directrices are $x = \pm \frac{a}{e}$

Hence distance between them is $2$.$\frac{6}{{2/3}} = 18$.

Standard 11
Mathematics

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