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10-2. Parabola, Ellipse, Hyperbola
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If the angle between the lines joining the end points of minor axis of an ellipse with its foci is $\pi\over2$, then the eccentricity of the ellipse is
A
$1\over2$
B
$1/\sqrt 2 $
C
$\sqrt 3 /2$
D
$1/2\sqrt 2 $
(IIT-1997)
Solution

(b) Since $\angle \,FBF' = \frac{\pi }{2}$ (Given)
$\therefore $$\angle \,FBC = \,\angle \,F'BC = \pi /4$
$CB = CF \Rightarrow \,\,b = ae$
==> ${b^2} = {a^2}{e^2}$
==> ${a^2}(1 – {e^2}) = {a^2}{e^2}$
==> $1 – {e^2} = {e^2}$
==> $2{e^2} = 1$
==> $e = 1/\sqrt 2 $.
Standard 11
Mathematics
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