JEE Main & Advanced Mathematics Definite Integration Question Bank Fundamental definite integration, Definite integration by substitution

  • question_answer
    The value of integral \[\int_{1/\pi }^{2/\pi }{\frac{\sin (1/x)}{{{x}^{2}}}}\,dx=\]  [IIT 1990]

    A)                 2             

    B)                 \[-1\]

    C)                 0             

    D)                 1

    Correct Answer: D

    Solution :

               Put \[t=\frac{1}{x}\Rightarrow dt=-\frac{1}{{{x}^{2}}}dx\]as \[t=\frac{\pi }{2}\]and \[\pi \]                    \[\therefore \] \[\int_{1/\pi }^{2/\pi }{\frac{\sin \left( \frac{1}{x} \right)}{{{x}^{2}}}dx}\]\[=-\int_{\pi /2}^{\pi }{\sin t\,dt=-[\cos t]_{\pi /2}^{\pi }}\]                                                     \[=-\left[ \cos \pi -\cos \left( \frac{\pi }{2} \right) \right]=1\].


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