You want to draw out the triangle - it will help.

is the angle such that . So if the opposite side of is and the hypotenuse is , what is the adjacent side to ?

Well, using the Pythagorean Theorem, we know that it equals . Now that we know the opposite and adjacent sides to , we can find , which equals .

So we know that:

Thus

Using an analogous method to this one, see if you can compute .