You may have identified the repeating pattern and seen that the points where the function equals 0 have odd factors and involve $\displaystyle\pi\usepackage{cmbright}\Huge\$ , but they are not odd multiples of $\displaystyle\pi\usepackage{cmbright}\Huge\$ . They are odd multiples of $\displaystyle\frac{\pi}{{2}}\usepackage{cmbright}\Huge\$ . For example, $\displaystyle{\cos{{3}}}\pi=-{1}\usepackage{cmbright}\Huge\$ and $\displaystyle{3}\pi\usepackage{cmbright}\Huge\$ is an odd multiple of $\displaystyle\pi\usepackage{cmbright}\Huge\$ . The correct answer is: all odd multiples of $\displaystyle\frac{\pi}{{2}}\usepackage{cmbright}\Huge\$