The definition of sin theta in any case is opposite/hypotenuse and cos theta is adjecent/hypotenuse. The unit circle has a radius of 1 so any right triangle with vertices at the origin, a point P on the circle, and the X or Y axis ( is a purely your choice, most choose the X axis ) will have a...
This is a lovely example of the special properties of the unit circle. The first thing you should do though with a problem like this is draw a picture.
http://img24.imageshack.us/my.php?image=tempw.jpg