The point $\displaystyle{\left(-\frac{{1}}{{2}},\frac{\sqrt{{3}}}{{2}}\right)}\usepackage{cmbright}\Huge\$ is where the angle $\displaystyle\frac{{{2}\pi}}{{3}}\usepackage{cmbright}\Huge\$ radians or $\displaystyle{{120}}^{\circ}\usepackage{cmbright}\Huge\$ intersects the unit circle. The x-coordinate gives you the value of the cosine function at that angle, and the y-coordinate gives you the value of the sine function at that angle. So:

$\displaystyle{\cos{{\left(\frac{{{2}\pi}}{{3}}\right)}}}=-\frac{{1}}{{2}}{\quad\text{and}\quad}{\sin{{\left(\frac{{{2}\pi}}{{3}}\right)}}}=\frac{\sqrt{{3}}}{{2}}\usepackage{cmbright}\Huge\$