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JEE Advanced · Mathematics · 24. Differentiation

If \(f^{\prime \prime}(x)=-f(x)\), where \(f(x)\) is a continuous double differentiable function and \(g(x)=f^{\prime}(x)\). If \(F(x)=\left(f\left(\frac{x}{2}\right)\right)^2+\left(g\left(\frac{x}{2}\right)\right)^2\) and \(F(5)=5\), then \(F(10)\) is

  1. A
    0
  2. B
    5
  3. C
    10
  4. D
    25
Verified Solution

Answer & Solution

Correct Answer

(B)
5

Step-by-step Solution

Detailed explanation

As, \(f(x)=-f(x) \Rightarrow \frac{d}{d x}\left(f^{\prime}(x)\right)=-f(x)\) \(\Rightarrow \quad g^{\prime}(x)=-f(x)\) and \(f^{\prime}(x)=g(x)\) where, \(\therefore \quad F^{\prime}(x)=2\left(f\left(\frac{x}{2}\right)\right) \cdot f^{\prime}\left(\frac{x}{2}\right) \cdot \frac{1}{2}+2\left(g\left(\frac{x}{2}\right)\right) \cdot g^{\prime}\left(\frac{x}{2}\right) \cdot \frac{1}{2}=0\) [using Eq. (1)] \(\therefore F(x)\) is constant \(\Rightarrow F(10)=F(5)=5\)
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