The second point is on the graph of cosine, but the first point is not. 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{{3}}}\pi={\cos{\pi}}=-{1}\usepackage{cmbright}\Huge\$

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