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10-2. Parabola, Ellipse, Hyperbola
normal
The eccentricity of an ellipse whose length of latus rectum is equal to distance between its foci, is
A
$2\,sin\,18^o$
B
$2\,cos\,36^o$
C
$sin\,18^o$
D
$cos\,36^o$
Solution
$\frac{2 b^{2}}{a}=2 a e $
$\Rightarrow a^{2}\left(1-e^{2}\right)=a^{2} e$
$\Rightarrow e^{2}+e-1=0 $
$\Rightarrow e=\frac{\sqrt{5}-1}{2}$
$=2 \sin 18^{\circ}$
Standard 11
Mathematics