When you draw an angle in standard position and it intersects the unit circle at the point $\displaystyle{\left({x},{y}\right)}\usepackage{cmbright}\Huge\$ , the x-coordinate gives you the value of the cosine function at that angle, and the y-coordinate gives you the value of the sine function at that angle. Because $\displaystyle\pi\usepackage{cmbright}\Huge\$ intersects the unit circle at the point $\displaystyle{\left(-{1},{0}\right)}\usepackage{cmbright}\Huge\$ , this means that $\displaystyle{\cos{\pi}}=-{1}{\quad\text{and}\quad}{\sin{\pi}}={0}\usepackage{cmbright}\Huge\$ .