If you draw a Sine graph, you will see that sin(x) is zero at 0, pi, 2*pi, ... If you draw a cosine graph you will see that it is -1 at pi. Therefore in the ab

If you draw a Sine graph, you will see that sin(x) is zero at 0, pi, 2*pi, ... If you draw a cosine graph you will see that it is -1 at pi. Therefore in the above sum the sine part just falls away. It is easy to see that the above is true when you recall that cosine(theta) = adjacent side/(hypotenuse). Since pi=180 degrees, adjacent will be -1 hypotenuse 1 and opposite side 0.