The graph of cosine has a repeating hill and valley pattern. The pattern repeats after intervals of length $\displaystyle{2}\pi\usepackage{cmbright}\Huge\$ in both directions. In particular:

$\displaystyle{\cos{\pi}}=-{1}\usepackage{cmbright}\Huge\$ , so $\displaystyle{\cos{{5}}}\pi={\cos{{3}}}\pi={\cos{\pi}}=-{1}\usepackage{cmbright}\Huge\$

So the points $\displaystyle{\left({3}\pi,-{1}\right)}\usepackage{cmbright}\Huge\$ and $\displaystyle{\left({5}\pi,-{1}\right)}\usepackage{cmbright}\Huge\$ are on the graph of cosine.