If you refer to the unit circle, since the y coordinate of the point cooresponds to the sine value, the sine of 180° is 0.
(Of course an 180° angle is unimaginable in a right triangle, but with a 0.00001° angle (which is the same as a 179.99999° angle because sinx = 180 - sinx (see unit circle)) you would have an infintesimally small opposite side and an infinitely large hypotenuse. The 1/∞ is pretty much 0 at that point, and 0/x = 0.)
