The effect of subtracting $\displaystyle\frac{\pi}{{2}}\usepackage{cmbright}\Huge\$ from the input of the function is to shift the graph of $\displaystyle{y}={\cos{\theta}}\usepackage{cmbright}\Huge\$ to the right by $\displaystyle\frac{\pi}{{2}}\usepackage{cmbright}\Huge\$ units. So, for example, the crossing point on the y-axis that was at $\displaystyle{\left(-\frac{\pi}{{2}},{0}\right)}\usepackage{cmbright}\Huge\$ in the graph of $\displaystyle{y}={\cos{\theta}}\usepackage{cmbright}\Huge\$ becomes a crossing point on the y-axis at $\displaystyle{\left({0},{0}\right)}\usepackage{cmbright}\Huge\$ , the origin. So the graph of $\displaystyle{y}={\cos{{\left(\theta-\frac{\pi}{{2}}\right)}}}\usepackage{cmbright}\Huge\$ will be the same as the graph of sine, which is the graph shown.