J & K CET Engineering J and K - CET Engineering Solved Paper-2004

  • question_answer
    If   \[f(x)\]  is   a   function   satisfying \[f\left( \frac{1}{x} \right)+{{x}^{2}}f(x)=0\]for all non-zero x, then \[\int_{\sin \,\theta }^{\text{cosec }\theta }{f(x)\,\,dx}\]is equal to

    A) \[0\]            

    B) \[1\]

    C) \[2\]            

    D) \[3\]

    Correct Answer: A

    Solution :

    Given that, \[f\left( \frac{1}{x} \right)+{{x}^{2}}\,f(x)=0\] ?.(i) Let \[I=\int_{\sin \,\theta }^{\text{cosec }\theta }{f(x)\,dx}\] Put   \[x=\frac{1}{t}\Rightarrow dx=-\frac{1}{{{t}^{2}}}dt\] \[\therefore \] \[I=-\int_{\cos ec\,\theta }^{\sin \,\theta }{f\left( \frac{1}{t} \right).\frac{1}{{{t}^{2}}}\,\,dt}\] \[=\int_{\cos ec\,\theta }^{\sin \,\theta }{\frac{1}{{{x}^{2}}}}f\left( \frac{1}{x} \right)\,dx\] \[=-\int_{\cos ec\,\theta }^{\sin \,\theta }{f(x)\,dx}\] [From Eq. (i)] \[\Rightarrow \] \[I=-I\] \[\Rightarrow \] \[I=0\]


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