What if the angle you are given is Cosine of 270 degrees?

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270 degrees is 3pi/2 radians.  3*pi/2 is negative along the y-axis (when looking at a circle centered on the origin).  Since Cos(theta) is the x-value of the (x,y) point on the unit circle at angle theta, we know that Cos(3*pi/2) = 0. (because along the y-axis, x is 0)