For any $\displaystyle-{1}<{y}<{1}\usepackage{cmbright}\Huge\$ , there are 4 values of $\displaystyle\theta\usepackage{cmbright}\Huge\$ in this interval that yield $\displaystyle{\cos{\theta}}={y}\usepackage{cmbright}\Huge\$ . But this is not true for $\displaystyle{y}={1}\usepackage{cmbright}\Huge\$ or $\displaystyle{y}=-{1}\usepackage{cmbright}\Huge\$ . The graph is symmetric about the y-axis, and the value of cosine is 1 when $\displaystyle\theta={0}\usepackage{cmbright}\Huge\$ . You need to find the other points where the value of cosine is 1.