J & K CET Engineering J and K - CET Engineering Solved Paper-2011

  • question_answer
    If \[g(x)\] is the inverse of \[f(x)\] and \[f'(x)=\cos x,\] then \[g'(x)\] is equal to

    A)  \[\sec \,\,x\]           

    B)  \[\sec \,\,(g\,\,(x))\]

    C)  \[\cos \,(g\,(x))\]      

    D)  \[-\sin \,\,(g\,(x)\,)\]

    Correct Answer: B

    Solution :

    Given, \[g(x)\] is the inverse of \[f(x)\] and \[f'(x)=cos\,\,x\] ?.(i) \[\Rightarrow \] \[g(x)={{f}^{-1}}(x)\] \[\Rightarrow \] \[x=f(g(x))\] On differentiating \[\Rightarrow \] \[1=f'\,(g(x)).g'(x)\] \[\Rightarrow \]\[g'(x)=\frac{1}{f'(g(x))}=\frac{1}{\cos \,(g(x))}\][from Eq.(i)] \[\Rightarrow \] \[g'(x)=sec\,g\,(x)\]


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