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. The maximum value of the cosine function is 1, and it occurs at the top of each hill in the graph. Perhaps you were looking at only the points $\displaystyle\theta={0}\usepackage{cmbright}\Huge\$ where the function equals 1 and $\displaystyle\theta=\pi\usepackage{cmbright}\Huge\$ where the function equals $\displaystyle-{1}\usepackage{cmbright}\Huge\$ , but in this interval there are other points where the function equals $\displaystyle-{1}\usepackage{cmbright}\Huge\$ and 1.