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JEE Main 2021
Trigonometry
Trigonometric Ratio and Identites
Medium

Question

If 2sinα1+cos2α=17{{\sqrt 2 \sin \alpha } \over {\sqrt {1 + \cos 2\alpha } }} = {1 \over 7} and 1cos2β2=110\sqrt {{{1 - \cos 2\beta } \over 2}} = {1 \over {\sqrt {10} }} α,β(0,π2)\alpha ,\beta \in \left( {0,{\pi \over 2}} \right) then tan(α\alpha + 2β\beta ) is equal to _____.

Answer: 2

Solution

2sinα1+cos2α=17{{\sqrt 2 \sin \alpha } \over {\sqrt {1 + \cos 2\alpha } }} = {1 \over 7} \Rightarrow 2sinα2cos2α{{\sqrt 2 \sin \alpha } \over {\sqrt {2{{\cos }^2}\alpha } }} = 17{1 \over 7} \Rightarrow 2sinα2cosα{{\sqrt 2 \sin \alpha } \over {\sqrt 2 \cos \alpha }} = 17{1 \over 7} \Rightarrow tanα\alpha = 17{1 \over 7} Also given 1cos2β2=110\sqrt {{{1 - \cos 2\beta } \over 2}} = {1 \over {\sqrt {10} }} \Rightarrow 2sinβ2{{\sqrt 2 \sin \beta } \over {\sqrt 2 }} = 110{1 \over {\sqrt {10} }} \Rightarrow sin β\beta = 110{1 \over {\sqrt {10} }} \therefore tan β\beta = 13{1 \over 3} tan2β=2tanβ1tan2β\tan 2\beta = {{2\tan \beta } \over {1 - {{\tan }^2}\beta }} = 2(13)119{{2\left( {{1 \over 3}} \right)} \over {1 - {1 \over 9}}} = 34{3 \over 4} \therefore tan(α\alpha + 2β\beta ) = tanα+tan2β1tanα.tan2β{{\tan \alpha + \tan 2\beta } \over {1 - \tan \alpha .\tan 2\beta }} = 17+34117.34{{{1 \over 7} + {3 \over 4}} \over {1 - {1 \over 7}.{3 \over 4}}} = 2525{{25} \over {25}} = 1

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