Perhaps you correctly recognized that there are three hills in the interval $\displaystyle{\left[-{3}\pi,{3}\pi\right]}\usepackage{cmbright}\Huge\$ , but thought that there was a single point on each hill where $\displaystyle{\sin{\theta}}=\frac{{1}}{{2}}\usepackage{cmbright}\Huge\$ , but this is not so. Notice that on the hill the value of $\displaystyle{\sin{\theta}}\usepackage{cmbright}\Huge\$ goes up from 0 to 1, and then down from 1 to 0.