The second point is on the graph of cosine, but the first point is not. The graph of the cosine function is a symmetric function about the y-axis. A function whose graph is symmetric about the y-axis has $\displaystyle{f{{\left(-{x}\right)}}}={f{{\left({x}\right)}}}\usepackage{cmbright}\Huge\$ , so $\displaystyle{\cos{{\left(-\pi\right)}}}={\cos{{\left(\pi\right)}}}=-{1}\usepackage{cmbright}\Huge\$ .