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JEE Main 2021
Trigonometric Equations
Trigonometric Equations
Medium

Question

The sum of solutions of the equation cosx1+sinx=tan2x{{\cos x} \over {1 + \sin x}} = \left| {\tan 2x} \right|, x(π2,π2){π4,π4}x \in \left( { - {\pi \over 2},{\pi \over 2}} \right) - \left\{ {{\pi \over 4}, - {\pi \over 4}} \right\} is :

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Solution

cosx1+sinx=tan2x{{\cos x} \over {1 + \sin x}} = \left| {\tan 2x} \right| cos2x/2sin2x/2(cosx/2+sinx/2)2=tan2x \Rightarrow {{{{\cos }^2}x/2 - {{\sin }^2}x/2} \over {(\cos x/2 + \sin x/2)^2}} = \left| {\tan 2x} \right| \Rightarrow cosx2sinx2cosx2+sinx2=tan2x{{\cos {x \over 2} - \sin {x \over 2}} \over {\cos {x \over 2} + \sin {x \over 2}}} = \left| {\tan 2x} \right| \Rightarrow 1tanx21+tanx2=tan2x{{1 - \tan {x \over 2}} \over {1 + \tan {x \over 2}}} = \left| {\tan 2x} \right| \Rightarrow tanπ4tanx2tanπ4+tanx2=tan2x{{\tan {\pi \over 4} - \tan {x \over 2}} \over {\tan {\pi \over 4} + \tan {x \over 2}}} = \left| {\tan 2x} \right| tan2(π4π2)=tan22x \Rightarrow {\tan ^2}\left( {{\pi \over 4} - {\pi \over 2}} \right) = {\tan ^2}2x 2x=nπ±(π4π2) \Rightarrow 2x = n\pi \pm \left( {{\pi \over 4} - {\pi \over 2}} \right) x=3π10,π6,π10 \Rightarrow x = {{ - 3\pi } \over {10}},{{ - \pi } \over 6},{\pi \over {10}} or sum =11π6 = {{ - 11\pi } \over 6}.

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