MCQ
$\int_0^{\pi / 6}\left(2+3 x^2\right) \cos 3 x d x=$
  • A
    $\frac{1}{36}(\pi+16)$
  • B
    $\frac{1}{36}(\pi-16)$
  • C
    $\frac{1}{36}\left(\pi^2-16\right)$
  • $\frac{1}{36}\left(\pi^2+16\right)$

Answer

Correct option: D.
$\frac{1}{36}\left(\pi^2+16\right)$
(D)
$\int_0^{\frac{\pi}{6}}\left(2+3 x^2\right) \cos 3 x d x$
$=\left[\left(2+3 x^2\right) \cdot \frac{\sin 3 x}{3}\right]_0^{\frac{\pi}{6}}-\int_0^{\frac{\pi}{6}} 6 x \cdot \frac{\sin 3 x}{3} d x$
$=\frac{2}{3}+\frac{\pi^2}{36}+\left[\frac{2 x \cos 3 x}{3}\right]_0^{\frac{\pi}{6}}-\frac{2}{3} \int_0^{\frac{\pi}{6}} \cos 3 x d x$
$\begin{array}{l}=\frac{2}{3}+\frac{\pi^2}{36}+0-\frac{2}{9}[\sin 3 x]_0^{\pi / 6} \\ =\frac{2}{3}+\frac{\pi^2}{36}-\frac{2}{9} \\ =\frac{1}{36}\left(\pi^2+16\right)\end{array}$

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