$$\int \frac { d x } { \cos ( x / a ) }$$
$\left(\ln(|\sin(\frac{x}{2a})+\cos(\frac{x}{2a})|)-\ln(|\cos(\frac{x}{2a})-\sin(\frac{x}{2a})|)\right)a+С$
$\frac{1}{\cos(\frac{x}{a})}$
