cos(-240°) = cos 120°; the "adjacent sides" and the "hypotenuses" are the same in both cases. The supplement of 120° is 60°, and 60° is a positive acute angle. Since cos(-240°) is negative, we need to write
cos(-240°) = -cos(60°).
Check: cos(60°)= 1/2, so - cos(60°) = -1/2, which is the value of cos(-240°).
