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Miscellaneous Exercise 4 · Q71

Q.Evaluate: ∫0111+x2sin⁡−1 ⁣(2x1+x2)dx\int_0^1 \dfrac{1}{1+x^2}\sin^{-1}\!\left(\dfrac{2x}{1+x^2}\right)dx

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Let x=tan⁡θ, dx=sec⁡2θ dθx=\tan\theta,\ dx=\sec^2\theta\,d\theta, 1+x2=sec⁡2θ1+x^2=\sec^2\theta. Then 2x1+x2=2tan⁡θsec⁡2θ=2sin⁡θcos⁡θ=sin⁡2θ\dfrac{2x}{1+x^2}=\dfrac{2\tan\theta}{\sec^2\theta}=2\sin\theta\cos\theta=\sin2\theta, so sin⁡−1 ⁣(2x1+x2)=sin⁡−1(sin⁡2θ)=2θ\sin^{-1}\!\big(\tfrac{2x}{1+x^2}\big)=\sin^{-1}(\sin2\theta)=2\theta (valid since 2θ∈[0,π/2]2\theta\in[0,\pi/2] for x∈[0,1]x\in[0,1]). Also dx1+x2=dθ\dfrac{dx}{1+x^2}=d\theta. …

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