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Exercise 5.3 · Q12

Q.Find dydx\frac{dy}{dx} in the following: y=sin⁡−1(1−x21+x2),0<x<1y = \sin^{-1} \left(\frac{1-x^2}{1+x^2}\right), 0 < x < 1

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Substituting x=tan⁡θx = \tan\theta simplifies yy to π2−2tan⁡−1x\dfrac{\pi}{2} - 2\tan^{-1}x for 0<x<10 < x < 1; differentiating gives dydx=−21+x2\dfrac{dy}{dx} = -\dfrac{2}{1+x^2}.

Recognising the Identity

As in the neighbouring cos⁡−1\cos^{-1} version of this expression, 1−x21+x2=cos⁡2θ\dfrac{1-x^2}{1+x^2} = \cos 2\theta under x=tan⁡θx = \tan\theta. Here the outer function is sin⁡−1\sin^{-1}, so cos⁡2θ\cos 2\theta must first be rewritten as a sine using the complementary-angle identity before the inverse sine can cancel it.

Step-by-Step Solution

1. Substitute x=tan⁡θx = \tan\theta.

Since 0<x<10 < x < 1, let θ=tan⁡−1x∈(0,π4)\theta = \tan^{-1}x \in \left(0, \tfrac{\pi}{4}\right). Then:

1−x21+x2=1−tan⁡2θ1+tan⁡2θ=cos⁡2θ⇒y=sin⁡−1(cos⁡2θ).\frac{1-x^2}{1+x^2} = \frac{1-\tan^2\theta}{1+\tan^2\theta} = \cos 2\theta \quad\Rightarrow\quad y = \sin^{-1}(\cos 2\theta).

2. Rewrite the cosine as a sine.

Using cos⁡ϕ=sin⁡ ⁣(π2−ϕ)\cos\phi = \sin\!\left(\tfrac{\pi}{2}-\phi\right) with ϕ=2θ\phi = 2\theta:

cos⁡2θ=sin⁡ ⁣(π2−2θ)⇒y=sin⁡−1 ⁣[sin⁡ ⁣(π2−2θ)].\cos 2\theta = \sin\!\left(\frac{\pi}{2}-2\theta\right) \quad\Rightarrow\quad y = \sin^{-1}\!\left[\sin\!\left(\frac{\pi}{2}-2\theta\right)\right].

3. Check the range.

Since θ∈(0,π4)\theta \in \left(0, \tfrac{\pi}{4}\right), 2θ∈(0,π2)2\theta \in \left(0, \tfrac{\pi}{2}\right), so:

π2−2θ∈(0,π2)⊂[−π2,π2],\frac{\pi}{2}-2\theta \in \left(0, \frac{\pi}{2}\right) \subset \left[-\frac{\pi}{2}, \frac{\pi}{2}\right], …

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