JEE Main & Advanced Mathematics Functions Question Bank Differentiability

  • question_answer
    Let \[f(x+y)=f(x)+f(y)\]and \[f(x)={{x}^{2}}g(x)\] for all \[x,y\in R\], where \[g(x)\] is continuous function. Then \[f'(x)\] is equal to

    A)            \[g'(x)\]

    B)            \[g(0)\]

    C)            \[g(0)+g'(x)\]

    D)            0

    Correct Answer: D

    Solution :

               We have \[f'(x)=\underset{h\to 0}{\mathop{\lim }}\,\frac{f(x+h)-f(x)}{h}\]\[=\underset{h\to 0}{\mathop{\lim }}\,\frac{f(x)+f(h)-f(x)}{h}\]                                 \[\left[ \because f(x+y)=f(x)+f(y) \right]\]                    \[=\underset{h\to 0}{\mathop{\lim }}\,\frac{f(h)}{h}=\underset{h\to 0}{\mathop{\lim }}\,\frac{{{h}^{2}}g(h)}{h}=0.g(0)=0\]                    \[[\because \,\,g\]is continuous therefore \[\underset{h\to 0}{\mathop{\lim }}\,g(h)=g(0)]\].


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