JEE Main & Advanced Mathematics Indefinite Integrals Question Bank Integration by Substitution

  • question_answer
    If \[\int{\frac{\cos 4x+1}{\cos x-\tan x}}dx=k\,\,\cos 4x+c\] then              [DCE 2005]

    A)                 \[k=-1/2\]

    B)                 \[k=-1/8\]

    C)                 \[k=-1/4\]

    D)                 None of these

    Correct Answer: B

    Solution :

                       \[\int{\frac{1+\cos 4x}{\cot x-\tan x}}dx=\int{\frac{2{{\cos }^{2}}2x}{{{\cos }^{2}}x-{{\sin }^{2}}x}}.\sin x\cos x\,\,dx\]                                 \[=\int{\cos 2x\sin 2x\,\,dx=-\frac{1}{8}\cos 4x+c}\],  \[\therefore \]\[k=-1/8\].


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