I think I understand the basic ideas of the sum and difference formulas, I just don't get how to break down say, pi/7 into a form that could be worked with. I could convert it to a degree then back again once I have my answer, but that seems like a lot more work than is necessary. If it were 100 degrees I'd just find some form of 100 degrees that I knew had an angle that was on the unit circle. 100 degrees doesn't seem to work very well but you get what I'm attempting to ask here. Also, can it only work if it is an angle that can be divided evenly like 120 degrees?