Question
Evaluate the following : $\int \cos ^3 x \cdot d x$

Answer

$\quad \cos 3 A=4 \cos ^3 A-3 \cos A$
$
\begin{aligned}
I & =\int \frac{1}{4}(\cos 3 x+3 \cos x) \cdot d x \\
& =\frac{1}{4}\left(\sin 3 x \cdot \frac{1}{3}+3 \cdot \sin x\right)+c \\
& =\frac{1}{12} \sin 3 x+\frac{3}{4} \sin x+c
\end{aligned}
$

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