JEE Main & Advanced Mathematics Differentiation Question Bank Differentiation by Substitution

  • question_answer
    If \[y=\sin x+{{e}^{x}},\]then \[\frac{{{d}^{2}}x}{d{{y}^{2}}}=\]                                  [Karnataka CET 1999; UPSEAT 2001; Kurukshetra CEE 2002]

    A)            \[{{(-\sin x+{{e}^{x}})}^{-1}}\]

    B)            \[\frac{\sin x-{{e}^{x}}}{{{(\cos x+{{e}^{x}})}^{2}}}\]

    C)            \[\frac{\sin x-{{e}^{x}}}{{{(\cos x+{{e}^{x}})}^{3}}}\]

    D)            \[\frac{\sin x+{{e}^{x}}}{{{(\cos x+{{e}^{x}})}^{3}}}\]

    Correct Answer: C

    Solution :

               \[y=\sin x+{{e}^{x}}\]Þ \[\frac{dy}{dx}=\cos x+{{e}^{x}}\]            Þ  \[\frac{dx}{dy}={{(\cos x+{{e}^{x}})}^{-1}}\]                                     ?..(i)            Again, \[\frac{{{d}^{2}}x}{d{{y}^{2}}}=-{{(\cos x+{{e}^{x}})}^{-2}}(-\sin x+{{e}^{x}})\frac{dx}{dy}\]. Substituting the value of \[\frac{dx}{dy}\] from (i),  \[\frac{{{d}^{2}}x}{d{{y}^{2}}}=\frac{(\sin x-{{e}^{x}})}{{{(\cos x+{{e}^{x}})}^{2}}}\,{{(\cos x+{{e}^{x}})}^{-1}}\]\[=\frac{\sin x-{{e}^{x}}}{{{(\cos x+{{e}^{x}})}^{3}}}\].


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