If two tangents drawn from a point ($\alpha$, $\beta$) lying on the ellipse 25x2 + 4y2 = 1 to the parabola y2 = 4x are such that the slope of one tangent is four times the other, then the value of (10$\alpha$ + 5)2 + (16$\beta$2 + 50)2 equals ___________.
Answer (integer)
2929
Solution
$\because(\alpha, \beta)$ lies on the given ellipse, $25 \alpha^{2}+4 \beta^{2}=1\quad\quad...(i)$
<br/><br/>
Tangent to the parabola, $y=m x+\frac{1}{m}$ passes through $(\alpha, \beta)$. So, $\alpha m^{2}-\beta m+1=0$ has roots $m_{1}$ and $4 m_{1}$,
<br/><br/>
$$
m_{1}+4 m_{1}=\frac{\beta}{\alpha} \text { and } m_{1} \cdot 4 m_{1}=\frac{1}{\alpha}
$$
<br/><br/>
Gives that $4 \beta^{2}=25 \alpha \quad\quad...(ii)$
<br/><br/>
from (i) and (ii)
<br/><br/>
$25\left(\alpha^{2}+\alpha\right)=1\quad\quad...(iii)$
<br/><br/>
Now, $(10 \alpha+5)^{2}+\left(16 \beta^{2}+50\right)^{2}$
<br/><br/>
$=25(2 \alpha+1)^{2}+2500(2 \alpha+1)^{2}$
<br/><br/>
$=2525\left(4 \alpha^{2}+4 \alpha+1\right)$ from equation (iii)
<br/><br/>
$=2525\left(\frac{4}{25}+1\right)$
<br/><br/>
$=2929$
About this question
Subject: Mathematics · Chapter: Conic Sections · Topic: Parabola
This question is part of PrepWiser's free JEE Main question bank. 146 more solved questions on Conic Sections are available — start with the harder ones if your accuracy is >70%.