Let the foci of a hyperbola $H$ coincide with the foci of the ellipse $E: \frac{(x-1)^2}{100}+\frac{(y-1)^2}{75}=1$ and the eccentricity of the hyperbola $H$ be the reciprocal of the eccentricity of the ellipse $E$. If the length of the transverse axis of $H$ is $\alpha$ and the length of its conjugate axis is $\beta$, then $3 \alpha^2+2 \beta^2$ is equal to
Solution
<p>$E: \frac{(x-1)^2}{100}+\frac{(y-1)^2}{75}=1$</p>
<p>$$\begin{aligned}
\text { Eccentricity of ellipse, } e_E & =\sqrt{1-\frac{b^2}{a^2}} \\
& =\sqrt{1-\frac{75}{100}} \\
& e_E=\frac{1}{2}
\end{aligned}$$</p>
<p>$\therefore e_H=2$ [ as eccentricity of hyperbola is reciprocal of eccentricity of ellipse]</p>
<p>Transverse axis of hyperbola $=\alpha$</p>
<p>Conjugate axis of hyperbola $=\beta$</p>
<p>Also, foci of ellipse $(1 \pm a e, 1)$</p>
<p>$$\begin{aligned}
& =\left(1 \pm\left(10 \times \frac{1}{2}\right), 1\right) \\
& =(1 \pm 5,1) \\
& =(6,1) \text { and }(-4,1)
\end{aligned}$$</p>
<p>Distance between foci $=10$</p>
<p>$$\begin{aligned}
& 2 a e=10 \\
& \Rightarrow a=\frac{5}{2}
\end{aligned}$$</p>
<p>$$\begin{aligned}
& \text { also, } e^2=1+\frac{b^2}{a^2} \\
& \begin{aligned}
4 & =1+\frac{4 b^2}{25} \\
b^2 & =\frac{75}{4} \\
b & =\frac{\sqrt{75}}{2}
\end{aligned} \\
& \Rightarrow \quad \alpha=5 \\
& \text { and } \beta=\sqrt{75} \\
& 3 \alpha^2+2 \beta^2=3(5)^2+2(75)=225
\end{aligned}$$</p>
About this question
Subject: Mathematics · Chapter: Conic Sections · Topic: Parabola
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