Example 8. Find the integral $\int \cos \frac{x}{4} d x$.
Solution.

$$
\int \cos \frac{x}{4} d x=\int \cos \frac{x}{4} 4 d\left(\frac{x}{4}\right)=4 \int \cos \frac{x}{4} d\left(\frac{x}{4}\right)=4 \sin \frac{x}{4}+C
$$