These are the points that are the farthest left and right in the interval $\displaystyle{\left[-\frac{\pi}{{2}},\frac{{{3}\pi}}{{2}}\right]}\usepackage{cmbright}\Huge\$ . You are looking for the highest point, where the value of the function is 1, and the lowest point, where the value of the function is $\displaystyle-{1}\usepackage{cmbright}\Huge\$ .