I am trying to use the half angle formula to solve these problems:

Sin(-7pi/12)

Sin(7pi/12)

cos(13pi/12)

cos(3pi/8)

here is what I did cos(3pi/8) = 1/2(1+cis13pi/4)= sqrt (2)+sqrt(3)/2 but the book shows a different answer.

for Sin(-7pi/12) I got sqrt(2)-sqrt(3)/2 I must be doing something wrong and I must have not understood what I read in the text. This area of the text seems more difficult than the rest of the material we have covered. Please explain the half angle formula so even I can understand it.

Thank you for your paitience!

Keith Stevens