JEE Main & Advanced Sample Paper JEE Main - Mock Test - 39

  • question_answer
    Let \[f(x)\] be differentiable function and symmetric about\[x=2\]. Then the value of \[\int\limits_{0}^{4}{\cos (\pi x)\,f'(x)\,dx}\] is

    A) \[3\]                    

    B)        \[2\]                      

    C) \[1\]                     

    D)        \[0\]

    Correct Answer: D

    Solution :

    [d] \[f(x)\] is symmetrical about the line \[x=2.\] \[\therefore \,\,\,\,\,\,\,\,\,\,\,\,f(2-x)=f(2+x)\] \[\therefore \,\,\,\,\,\,\,\,\,\,\,\,f(x)=f(4-x)\]   (Replacing x by \[2-x\]) \[\Rightarrow \,\,\,\,\,\,\,\,\,\,\,\,\,\,f'(x)=f'(4-x)\] \[\Rightarrow \,\,\,\,\,\,\,\,\,\,\,I=\int\limits_{0}^{4}{\cos \,(\pi x)f'(x)\,dx}\] \[=\int\limits_{0}^{4}{\cos \,(4\pi -\pi x)\,f'(4-x)\,dx=-I\,\,\,\Rightarrow 2I=0}\]


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