The graph of $\displaystyle{y}={\cos{\theta}}\usepackage{cmbright}\Huge\$ is symmetric about the y-axis. This means that if you have one point on the graph, its mirror image over the y-axis is also on the graph. The mirror image of $\displaystyle{\left(\frac{{{7}\pi}}{{4}},\frac{\sqrt{{2}}}{{2}}\right)}\usepackage{cmbright}\Huge\$ is $\displaystyle{\left(-\frac{{{7}\pi}}{{4}},\frac{\sqrt{{2}}}{{2}}\right)}\usepackage{cmbright}\Huge\$ , so it is on the graph.