$\int \cos ^{3} x d x=?$

$\frac{1}{12} \sin 3 x+C$

$\frac{1}{12} \sin 3 x+\sin x+C$

$\frac{1}{12} \sin 3 x+9 \sin x+C$

$\frac{1}{12}(\sin 3 x+9 \sin x)+C$