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10-2. Parabola, Ellipse, Hyperbola
normal
The normal to the rectangular hyperbola $xy = c^2$ at the point $'t_1'$ meets the curve again at the point $'t_2'$ . Then the value of $t_{1}^{3} t_{2}$ is
A
$1$
B
$c$
C
$-c$
D
$-1$
Solution
Normal at $t_{1}$ meets the curve again at $t_{2}.$
So normal passes through point $\left(c t_{1}, c / t_{1}\right)$ and $\left(c t_{2}, c / t_{2}\right).$
Slope of normal
$=\frac{c / t_{2}-c / t_{1}}{c t_{2}-c t_{1}}=-\frac{1}{t_{1} t_{2}}$
Slope of normal from equation of normal at $t_{1}=t_{1}^{2}$
So $t_{1}^{2}=-\frac{1}{t_{1} t_{2}} \Rightarrow t_{1}^{3} t_{2}=-1$
Standard 11
Mathematics