This is one of the low points, but there is another. The graph of cosine has a valley, a hill, and a valley on the interval $\displaystyle{\left[-\frac{{{7}\pi}}{{2}},-\frac{\pi}{{2}}\right]}\usepackage{cmbright}\Huge\$ and the low points are in the middle of the valleys. The correct answer is: $\displaystyle{\left(-{3}\pi,-{1}\right)}\usepackage{cmbright}\Huge\$ and $\displaystyle{\left(-\pi,-{1}\right)}\usepackage{cmbright}\Huge\$