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

  • question_answer
    \[\int_{{}}^{{}}{\cos \sqrt{x}\ dx=}\]       [BIT Ranchi 1990; IIT 1977; RPET 1999]

    A)                 \[2[\sqrt{x}\sin \sqrt{x}+\cos \sqrt{x}]+c\]         

    B)                 \[2[\sqrt{x}\sin \sqrt{x}-\cos \sqrt{x}]+c\]

    C)                 \[2[\cos \sqrt{x}-\sqrt{x}\sin \sqrt{x}]+c\]          

    D)                 \[-2[\sqrt{x}\sin \sqrt{x}+\cos \sqrt{x}]+c\]

    Correct Answer: A

    Solution :

                    Put \[\sqrt{x}=t\Rightarrow \frac{1}{2\sqrt{x}}\,dx=dt\Rightarrow dx=2t\,dt,\] then it reduces to \[\int_{{}}^{{}}{2t\,.\cos t\,dt}=2\left[ t\,.\,\sin t-\int_{{}}^{{}}{\sin t\,dt} \right]\]                                 \[=2t\sin t+2\cos t\]\[=2[\sqrt{x}\sin \sqrt{x}+\cos \sqrt{x}]+c\].


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