(a) We're told that: .1) If
We have a right triangle with angle and:
Using Pythagorus, we find that: .
Since is in Quadrant 1, is in Quadrant 1.
. . Hence: .
Double-angle Identity: .
(b) Half-angle Identity: .
We are given: .2. If
Since is in Quadrant 2: . . . . Then: .