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Let $$f$$ be a continuous function satisfying $$\int\limits_{0}^{t^{2}}\left(f(x)+x^{2}\right) d x=\frac{4}{3} t^{3}, \forall t > 0$$. Then $$f\left(\frac{\pi^{2}}{4}\right)$$ is equal to :

JEE · Math · previous-year question

  1. A.$$-\pi\left(1+\frac{\pi^{3}}{16}\right)$$
  2. B.$$\pi\left(1-\frac{\pi^{3}}{16}\right)$$correct
  3. C.$$-\pi^{2}\left(1+\frac{\pi^{2}}{16}\right)$$
  4. D.$$\pi^{2}\left(1-\frac{\pi^{2}}{16}\right)$$

Answer

B. $$\pi\left(1-\frac{\pi^{3}}{16}\right)$$

Explanation

Given that $$ \int\limits_0^{t^2}\left(f(x)+x^2\right) d x=\frac{4}{3} t^3, \forall t>0 $$ On differentiating using Leibnitz rule, we get $$ \begin{aligned} & \left(f\left(t^2\right)+t^4\right) \times 2 t=\frac{4}{3} \times 3 t^2 \\\\ & \Rightarrow f\left(t^2\right)+t^4=2 t \\\\ & \Rightarrow f\left(t^2\right)=2 t-t^4 \end{aligned} $$ On substituting $\frac{\pi}{2}$ for $t$, we get $$ f\left(\frac{\pi^2}{4}\right)=2\left(\frac{\pi}{2}\right)-\frac{\pi^4}{16}=\pi\left(1-\frac{\pi^3}{16}\right) $$

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