cosine represents the x-value of the point on the unit circle that is [tex]\theta[/tex] degrees counterclockwise from the increasing x direction.
Then cosine is given by the hypotenuse over the adjacent side to the angle.
Then cos(<BAC) is simply [tex]\frac{4}{4\sqrt{5}}[/tex]. This can be simplified to [tex]\frac{\sqrt{5}}{5}[/tex].
