JEE Main & Advanced Mathematics Functions Question Bank Differentiability

  • question_answer
    The function \[f(x)=({{x}^{2}}-1)|{{x}^{2}}-3x+2|+\cos (|x|)\] is not differentiable at [IIT 1999]

    A)            ?1

    B)            0

    C)            1

    D)            2

    Correct Answer: D

    Solution :

               Since function \[|x|\] is not differentiable at \[x=0\]                    \[\therefore \,|{{x}^{2}}-3x+2|=|(x-1)(x-2)|\]                    Hence is not differentiable at \[x=1\] and 2                    Now\[f(x)=({{x}^{2}}-1)|{{x}^{2}}-3x+2|\cos (|x|)\] is not differentiable at \[x=2\]                    For \[1<x<2\], \[f(x)=-({{x}^{2}}-1)({{x}^{2}}-3x+2)+\cos x\]                    For \[2<x<3\], \[f(x)=+({{x}^{2}}-1)({{x}^{2}}-3x+2)+\cos x\]                    \[Lf'(x)=-({{x}^{2}}-1)(2x-3)-2x({{x}^{2}}-3x+2)-\sin x\]                    \[Lf'(2)=-3-\sin 2\]                    \[Rf'(x)=({{x}^{2}}-1)(2x-3)+2x({{x}^{2}}-3x+2)-\sin x\]                    \[Rf'(2)=(4-1)(4-3)+0-\sin 2=3-\sin 2\]                    Hence \[Lf'(2)\ne Rf'(2)\].


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