You may have identified the repeating pattern and seen that the points where the function equals 0 are multiples of $\displaystyle\frac{\pi}{{2}}\usepackage{cmbright}\Huge\$ , but they are not all the multiples of $\displaystyle\frac{\pi}{{2}}\usepackage{cmbright}\Huge\$ . For example, $\displaystyle{\cos{\pi}}=-{1}\usepackage{cmbright}\Huge\$ and $\displaystyle\pi\usepackage{cmbright}\Huge\$ is an even multiple of $\displaystyle\frac{\pi}{{2}}\usepackage{cmbright}\Huge\$ .