JEE Main & Advanced JEE Main Paper (Held on 12-4-2019 Afternoon)

  • question_answer
    Let \[\alpha \in (0,\pi /2)\] be fixed. If the integral \[\int_{{}}^{{}}{\frac{\tan x+\tan \alpha }{\tan x-\tan \alpha }dx=}\]\[A(x)cos2\alpha +B(x)sin2\alpha +C,\]  where C is a constant of integration, then the functions A(x) and B(x) are respectively : [JEE Main 12-4-2019 Afternoon]

    A) \[x\alpha \,\text{and lo}{{\text{g}}_{e}}|cos(x\alpha )|\]

    B) \[x+\alpha \,\text{and lo}{{\text{g}}_{e}}|sin(x\alpha )|\]

    C) \[x\alpha \,\text{and lo}{{\text{g}}_{e}}|sin(x\alpha )|\]

    D) \[x+\alpha \,\text{and lo}{{\text{g}}_{e}}|sin(x+\alpha )|\]

    Correct Answer: C

    Solution :

    \[\int_{{}}^{{}}{\frac{\tan x+\tan \alpha }{\tan x-\tan \alpha }}dx=\int_{{}}^{{}}{\frac{\sin \left( x+\alpha  \right)}{\sin \left( x-\alpha  \right)}}dx\]             Let\[x-\alpha =t\]          \[\Rightarrow \int_{{}}^{{}}{\frac{\sin \left( t+2\alpha  \right)}{\sin t}}dt=\int_{{}}^{{}}{\cos 2\alpha dt}+\int_{{}}^{{}}{\cot (t)sin2\alpha dt}\]             \[=t.\cos 2\alpha +\ell n\left| \sin t \right|.\sin 2\alpha +C\]             \[=(x-\alpha )cos2\alpha +\ln |\sin (x-\alpha )|.sin2\alpha +C\]


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