cos a = adjacent / hypotenuse
r = radius
cos a = x / r
To find the radius we use the following
[tex]r = \sqrt{x^2 + y^2}[/tex]
[tex]r = \sqrt{-2^2 + 3^2}[/tex]
[tex]r = \sqrt{13}[/tex]
[tex]\cos \alpha = \frac{x}{r}[/tex]
[tex]\cos \alpha = \frac{-2}{\sqrt{13}}[/tex]
[tex]\cos \alpha = -\frac{2\sqrt{13}}{13}[/tex]
