Question

Let a hyperbola passes through the focus of the ellipse $16 x^2+25 y^2=400$. The transverse and conjugate axes of this hyperbola coincide with the major and minor axes of the given ellipse. The eccentricity of the hyperbola is reciprocal of that the ellipse.
Which one of the following statement is correct?

• A. Vertices of hyperbola are $( \pm 3,0)$.
• B. Distance between foci of hyperbola is 6 .
• C. Equation of directrices of hyperbola are $x= \pm \frac{5}{9}$.
• D. None

Answer 1

Answer: (A)

For the given ellipse, $\frac{ x ^2}{25}+\frac{ y ^2}{16}=1, e =\sqrt{1-\frac{16}{25}}=\frac{3}{5}$.
So, eccentricity of hyperbola $=\frac{5}{3}$.
Let the hyperbola be, $\frac{ x ^2}{ A ^2}-\frac{ y ^2}{ B ^2}=1 \ldots$ (1)
Then, $B^2=A^2\left(\frac{25}{9}-1\right)=\frac{16}{9} A^2$. Also, foci of ellipse are $( \pm 3,0)$.
As, hyperbola passes through $( \pm 3,0)$. So, $\frac{9}{A^2}=1 \Rightarrow A^2=9, B^2=16$
$\Rightarrow$ Equation of hyperbola is $\frac{x^2}{9}-\frac{y^2}{16}=1$
Vertices of hyperbola are $( \pm 3,0) \Rightarrow( A )$ is correct.
Focal length of hyperbola $=10 \Rightarrow( B )$ is incorrect.
Equation of directrices of hyperbola are $x= \pm \frac{9}{5} . \Rightarrow(C)$ is incorrect.