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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