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Question: Answered & Verified by Expert
If $x=\frac{1-t^2}{1+t^2}$ and $y=\frac{2 a t}{1+t^2}$, then $\frac{d y}{d x}=$
MathematicsDifferentiationMHT CETMHT CET 2021 (22 Sep Shift 1)
Options:
  • A $\frac{\mathrm{a}\left(\mathrm{t}^2+1\right)}{2 \mathrm{t}}$
  • B $\frac{a\left(t^2-1\right)}{t}$
  • C $\frac{a\left(1-t^2\right)}{2 t}$
  • D $\frac{a\left(t^2-1\right)}{2 t}$
Solution:
2832 Upvotes Verified Answer
The correct answer is: $\frac{a\left(t^2-1\right)}{2 t}$
We have $\mathrm{x}=\frac{1-\mathrm{t}^2}{1+\mathrm{t}^2}$ and $\mathrm{y}=\frac{2 \mathrm{at}}{1+\mathrm{t}^2}$
Put $\mathrm{t}=\tan \theta \Rightarrow \mathrm{x}=\cos 2 \theta$ and $\mathrm{y}=\mathrm{a} \sin 2 \theta$
$$
\frac{d x}{d \theta}=-2 \sin 2 \theta \text { and } \frac{d y}{d \theta}=2 a \cos 2 \theta
$$
$$
\therefore \frac{d y}{d x}=\frac{2 a \cos 2 \theta}{-2 \sin 2 \theta}=\frac{-a}{\tan 2 \theta}=\frac{-a}{\left(\frac{2 \tan \theta}{1-\tan ^2 \theta}\right)}
$$
$$
=\frac{-\mathrm{a}}{\left(\frac{2 \mathrm{t}}{1-\mathrm{t}^2}\right)}=\frac{-\mathrm{a}\left(1-\mathrm{t}^2\right)}{2 \mathrm{t}}=\frac{\mathrm{a}\left(\mathrm{t}^2-1\right)}{2 \mathrm{t}}
$$

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