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\$ . The first valley covers the interval from $\displaystyle\theta=-\frac{{{7}\pi}}{{2}}\usepackage{cmbright}\Huge\$ to $\displaystyle\theta=-\frac{{{5}\pi}}{{2}}\usepackage{cmbright}\Huge\$ and the second valley covers the interval from $\displaystyle\theta=-\frac{{{3}\pi}}{{2}}\usepackage{cmbright}\Huge\$ to $\displaystyle\theta=-\frac{\pi}{{2}}\usepackage{cmbright}\Huge\$ . The low points are in the middle of the valleys: $\displaystyle{\left(-{3}\pi,-{1}\right)}\usepackage{cmbright}\Huge\$ and $\displaystyle{\left(-\pi,-{1}\right)}\usepackage{cmbright}\Huge\$ .