$\displaystyle\pi\usepackage{cmbright}\Huge\$ radians corresponds to the point $\displaystyle{\left(-{1},{0}\right)}\usepackage{cmbright}\Huge\$ on the unit circle. The coordinates of this point give you the values of $\displaystyle{\cos{\pi}}\usepackage{cmbright}\Huge\$ and $\displaystyle{\sin{\pi}}\usepackage{cmbright}\Huge\$ . The first coordinate of $\displaystyle{\left(-{1},{0}\right)}\usepackage{cmbright}\Huge\$ tells you that $\displaystyle{\cos{\pi}}=-{1}\usepackage{cmbright}\Huge\$ , and so $\displaystyle{\left(\pi,-{1}\right)}\usepackage{cmbright}\Huge\$ is on the graph of $\displaystyle{y}={\cos{\theta}}\usepackage{cmbright}\Huge\$ . The value of the sine function is given by the second coordinate of the points on the unit circle.