Gujarati
10-2. Parabola, Ellipse, Hyperbola
hard

The foci of the ellipse $25{(x + 1)^2} + 9{(y + 2)^2} = 225$ are at

A

$(-1, 2)$ and $(-1, -6)$

B

$(-1, 2)$ and $(6, 1)$

C

$(1, -2)$ and $(1, -6)$

D

$(-1, -2)$ and $(1, 6)$

Solution

(a) $\frac{{{{(x + 1)}^2}}}{{\frac{{225}}{{25}}}} + \frac{{{{(y + 2)}^2}}}{{\frac{{225}}{9}}} = 1$

$a = \sqrt {\frac{{225}}{{25}}} = \frac{{15}}{5},\,b = \sqrt {\frac{{225}}{9}} = \frac{{15}}{3}$

==> $e = \sqrt {1 – \frac{9}{{25}}} = \frac{4}{5}$

Focus $ = \left( { – 1, – 2 \pm \frac{{15}}{3}.\frac{4}{5}} \right) $

$= ( – 1, – 2 \pm 4)$

$=(-1,2); (-1,-6) .$

Standard 11
Mathematics

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