We start off with x and y axes:



Then we start our angle from the right side of the x-axis, and

since the angle is negative, we swing an arc in the clockwise
direction through 330° and draw a radius vector:



And we see that the angle is coterminal with 30°, since one
revolution is 360° and 330° subtracted from 360° is 30°,
and so the reference angle is 30°, 






and since -330° has its
terminal side in the first quadrant, its cosine will be the
same as the cosine of 30° and positive. so:



Edwin