Question
Prove that $\sin^{–1} \Bigg(\frac{8}{17}\Bigg)+\sin^{-1}\Bigg(\frac{3}{5}\Bigg)=\cos^{-1}\Bigg(\frac{36}{85}\Bigg).$

Answer

Writing $\sin^{-1 }\Bigg(\frac{18}{17}\Bigg)=\tan^{-1}\frac{8}{15}$ and $\sin^{-1 }\Bigg(\frac{3}{5}\Bigg)=\tan^{-1}\frac{3}{4}$
$\therefore LHS = \tan^{-1 }\frac{8}{15}+\tan^{-1}\frac{3}{4}=\tan^{-1}\Bigg(\frac{\frac{8}{15}+\frac{3}{4}}{1-\frac{8}{15}\cdot\frac{3}{4}}\Bigg)=\tan^{1}\Bigg(\frac{77}{36}\Bigg)$
Getting $\tan^{–1 }\Bigg(\frac{77}{36}\Bigg)=\cos^{-1}\Bigg(\frac{36}{85}\Bigg).$

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