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10-2. Parabola, Ellipse, Hyperbola
hard
Let the eccentricity of the hyperbola $H : \frac{ x ^{2}}{ a ^{2}}-\frac{ y ^{2}}{ b ^{2}}=1$ be $\sqrt{\frac{5}{2}}$ and length of its latus rectum be $6 \sqrt{2}$, If $y =2 x + c$ is a tangent to the hyperbola $H$, then the value of $c ^{2}$ is equal to
A
$18$
B
$20$
C
$24$
D
$32$
(JEE MAIN-2022)
Solution
$y = mx \pm \sqrt{ a ^{2} m ^{2}- b ^{2}}$
$m =2, c ^{2}= a ^{2} m ^{2}- b ^{2}$
$c ^{2}=4 a ^{2}- b ^{2}$
$e ^{2}=1+\frac{ b ^{2}}{ a ^{2}}$
$\frac{5}{2}=1+\frac{b^{2}}{a^{2}}$
$\frac{3}{2}=\frac{b^{2}}{a^{2}} \Rightarrow b^{2}=\frac{3 a^{2}}{2}$
$\frac{2 b^{2}}{a}=6 \sqrt{2}$
$\frac{2}{a} \times \frac{3 a^{2}}{2}=6 \sqrt{2}$
$3 a=6 \sqrt{2}$
$a=2 \sqrt{2}$
$b^{2}=\frac{3}{2} \times 8=12$
$b=2 \sqrt{3}$
$\therefore c^{2}=4 \times 8-12$
$c^{2}=20$
Standard 11
Mathematics