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. Perhaps you were looking at only the point $\displaystyle\theta=\pi\usepackage{cmbright}\Huge\$ , but there are other points on the interval where the function equals $\displaystyle-{1}\usepackage{cmbright}\Huge\$ .