Perhaps you thought of 0 as the minimum value, but actually $\displaystyle-{1}\usepackage{cmbright}\Huge\$ is the minimum value. The function attains this value at the bottom of every valley on the graph. You need to find the values of $\displaystyle\theta\usepackage{cmbright}\Huge\$ that correspond to the bottoms of the valleys on the interval $\displaystyle{\left[\pi,{4}\pi\right]}\usepackage{cmbright}\Huge\$ .