JEE Main & Advanced Mathematics Limits, Continuity and Differentiability Question Bank Self Evaluation Test - Continuity and Differentiability

  • question_answer
    Let \[f(x+y)=f(x)+f(y)\] and \[f(x)={{x}^{2}}g(x)\] for all\[x,\text{ }y\in R\], 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 :

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


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