Question

Find an anti derivative (or integral) of the given function by the method of inspection.
$\cos 3x$

Answer 1

We know that,
$\frac{d}{dx}\left(\sin 3x\right)=3\cos 3x$
$\therefore \cos 3x=\frac{d}{dx}\left(\frac{1}{3}\sin 3x\right)$
Therefore, the anti derivative of $\cos 3x$ is $\frac{1}{3}\sin 3x$