The minimum value of the cosine function is $\displaystyle-{1}\usepackage{cmbright}\Huge\$ , and it occurs at the bottom of each valley in the graph. In the interval $\displaystyle{\left[-{2}\pi,{2}\pi\right]}\usepackage{cmbright}\Huge\$ , there are two valleys: at $\displaystyle\theta=-\pi\usepackage{cmbright}\Huge\$ and $\displaystyle\pi\usepackage{cmbright}\Huge\$ .