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Question 39 of 43

Q.If sin⁻¹x = tan⁻¹y, show that 1/x² - 1/y² = 1.

West Bengal WbchseWest Bengal HS (WBCHSE) Board 2025Subjective· 2mImportance★★★★★
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Set both inverse-trig expressions equal to a common angle θ\theta, then convert to a Pythagorean identity.

Let sin⁡−1x=tan⁡−1y=θ\sin^{-1}x=\tan^{-1}y=\theta. Then x=sin⁡θx=\sin\theta and y=tan⁡θy=\tan\theta.

1x2−1y2=1sin⁡2θ−1tan⁡2θ=1sin⁡2θ−cos⁡2θsin⁡2θ=1−cos⁡2θsin⁡2θ\frac{1}{x^2}-\frac{1}{y^2} = \frac{1}{\sin^2\theta}-\frac{1}{\tan^2\theta} = \frac{1}{\sin^2\theta}-\frac{\cos^2\theta}{\sin^2\theta} = \frac{1-\cos^2\theta}{\sin^2\theta}

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