JEE Main & Advanced Sample Paper JEE Main Sample Paper-2

  • question_answer
    The function \[f(x)={{(4{{\sin }^{2}}x-1)}^{n}}({{x}^{2}}-x+1),\,n\in N\] has local minimum at \[x=\frac{\pi }{6},\,\] then n is

    A)  even                    

    B)  odd

    C)  any natural number      

    D)  None of these

    Correct Answer: A

    Solution :

    \[f(x)={{(2\sin x+1)}^{n}}{{(2\sin x-1)}^{n}}({{x}^{2}}-x+1)\] If n  is odd \[f(x)\] changes sign about \[x=\frac{\pi }{6}\]. i.e., \[f\left( \frac{\pi }{6}+h \right)>f\left( \frac{\pi }{6} \right)\] and \[f\left( \frac{\pi }{6}-h \right)<f\left( \frac{\pi }{6} \right)\] \[\Rightarrow \] \[f(x)\] has neither maxima nor minima at \[\pi /6\].


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