The graph of cosine has a hill with endpoints at $\displaystyle\theta=-\frac{\pi}{{2}}\usepackage{cmbright}\Huge\$ and $\displaystyle\theta=\frac{\pi}{{2}}\usepackage{cmbright}\Huge\$ , where the value of the function is 0. It has a repeating hill and valley pattern in both directions, with each hill and valley having a width of $\displaystyle\pi\usepackage{cmbright}\Huge\$ units. So the endpoints of the hills and valleys, where the function is equal to 0, are:

$\displaystyle\theta=\pm\frac{\pi}{{2}},\pm\frac{{{3}\pi}}{{2}},\pm\frac{{{5}\pi}}{{2}},\pm\frac{{{7}\pi}}{{2}},\ldots\usepackage{cmbright}\Huge\$

These numbers are all odd multiples of $\displaystyle\frac{\pi}{{2}}\usepackage{cmbright}\Huge\$