In 10-3 we have a lot of formulas to remember.
1. sin2(alpha) = 2sin(alpha)cos(beta)
2. cos2(alpha) = cos^2(alpha)-sin^2(alpha)
3. cos2(alpha) = 1-2sin^2(alpha)
4. cos2(alpha) = 2cos^2(alpha)-1
5. tan2(alpha) = 2tan(alpha)/1-tan^2(alpha)
6.sin(alpha)/2 = +or- square root of 1-cos(alpha)/2
7. cos(alpha)/2 = +or- square root of 1+cos(alpha)/2
8. tan(alpha)/2 = +or- square root of 1-cos(alpha)/1+cos(alpha)
9. tan(alpha)/2 = sin(alpha)/1+cos(alpha)
10. tan(alpha)/2 = 1-cos(alpha)/sin(alpha)

When you are solving these types of problems:
1. Look at the problem and figure out which of the formulas you need to solve it. 
2. Fill in "alpha" and "beta" with the numbers that you are given. 
3. Simplify the equation until it can not be simplified any longer.