Quandaries
and Queries 

Who is asking: Student Level: Secondary Question: sin A + sin B = 2sin(A+B/2)cos(AB/2) cos A  cos B = 2sin(A+B/2)sin(AB/2) cos A + cos B = 2cos(A+B/2)cos(AB/2) sin A  sin B = 2cos(A+B/2)sin(A+B/2) Please help im almost there with these babies and it's very frustrating! 

Hi, I will illustrate how to derive the first expression. To do it I am going to assume that you know that
and that
From these I can derive
Adding equation 1 and equation 2 I get
thus
If you now let P = ^{A + B}/_{2} and Q = ^{A  B}/_{2} you will se that this is exactly the expression in your first problem. The other can be derived in a similar fashion. Penny 

