You may have recognized that all the points where the function equals 0 somehow involve $\displaystyle\pi\usepackage{cmbright}\Huge\$ , but this is not a description of those points. The cosine function equals 1 at all of the even multiples of $\displaystyle\pi\usepackage{cmbright}\Huge\$ , and it equals $\displaystyle-{1}\usepackage{cmbright}\Huge\$ at all of the odd multiples of $\displaystyle\pi\usepackage{cmbright}\Huge\$ . The correct answer is: all odd multiples of $\displaystyle\frac{\pi}{{2}}\usepackage{cmbright}\Huge\$