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

  • question_answer
    If \[f''(x)=-f(x)\] and \[g(x)=f'(x)\] and \[F(x)={{\left( f\left( \frac{x}{2} \right) \right)}^{2}}+{{\left( g\left( \frac{x}{2} \right) \right)}^{2}}\] and given that \[F(5)=5\], then F (10) is equal to-

    A) 5

    B) 10

    C) 0

    D) 15

    Correct Answer: A

    Solution :

    [a] \[F'(x)=\left[ f\left( \frac{x}{2} \right).f'\left( \frac{x}{2} \right)+g\left( \frac{x}{2} \right)g'\left( \frac{x}{2} \right) \right]\] Here, \[g(x)=f'(x)\] and \[g'(x)=f''(x)=-f(x)\] so \[F'(x)=f\left( \frac{x}{2} \right)g\left( \frac{x}{2} \right)-f\left( \frac{x}{2} \right)g\left( \frac{x}{2} \right)=0\] \[\Rightarrow F(x)\] is constant function so \[F(10)=5\]


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