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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