88. $\int \cos 5 x d x$

88. $\int \cos 5x \, dx$
Solution. According to the remark made earlier, the argument of the integrand should be under the differential sign. Therefore,

$$
\int \cos 5 x d x=\frac{1}{5} \int \cos 5 x d(5 x)=\frac{1}{5} \sin 5 x+C
$$