This graph does have the symmetric shape of a cosine function and it has the correct amplitude. However, it does not have the correct period. Remember that the period of $\displaystyle{y}={a}{\cos{{b}}}{x}\usepackage{cmbright}\Huge\$ is given by $\displaystyle\frac{{{2}\pi}}{{\left|{b}\right|}}\usepackage{cmbright}\Huge\$ .