Exam

BITSAT

Year

2013

Subject

Mathematics

Chapter

Ellipse

BITSAT 2013 · Mathematics · Ellipse

S and T are the foci of an ellipse and B is an end of the minor...

$\mathrm{S}$ and $\mathrm{T}$ are the foci of an ellipse and $\mathrm{B}$ is an end of the minor axis. If STB is an equilateral triangle, then the eccentricity of the ellipse is

Step-by-step solution

Let eq. of ellipse be $\frac{x^{2}}{a^{2}}+\frac{y^{2}}{b^{2}}=1$, $\mathrm{S}$ is $(-\mathrm{ae}, 0), \mathrm{T}$ is $(\mathrm{ae}, 0)$ and $\mathrm{B}$ is $(0, \mathrm{ b})$
$\Rightarrow \mathrm{SB}=\sqrt{(0+\mathrm{ae})^{2}+\mathrmb^{2}}$ Also $\mathrm{SB}^{2}=\mathrm{ST}^{2} \Rightarrow 4 \mathrma^{2} \mathrme^{2}=\mathrma^{2} \mathrme^{2}+\mathrmb^{2}\Rightarrow 3 a^{2} e^{2}=a^{2}\left(1-e^{2}\right)=a^{2}-a^{2} e^{2}\Rightarrow 4 a^{2} e^{2}=a^{2} \Rightarrow e^{2}=\frac{1}{4} \Rightarrow e=\frac{1}{2}$