Question

how do you figure out cos240°

Answer 1

well cos(240) is a number. Or are you trying to use that with something?

Answer 2

240 = 180 +60. Since you know the sin and cos of 180 and of 60, you could use the angle addition formula.

Answer 3

I find it easier to just use 60 and since you know the quadrant its in you can assign the positive or negave value where needed. Kind of shortcut I guess.