Amaths - double angles

Q1.
sin (A + B) =2sinAcosB only if A = B. There is no condition for the range of A, A can be any angle such as 100, 200, 300, -400 etc.
Q2.
cos(180 - x) = -cos x for 90 < x < 180. since x is in the 2nd quadrant.

2008-09-03 07:30:42 補充：
For example: cos100 = cos(180 - 80) = -cos 80. You can verify with your calculator.

Sin(A+B)=sinAcosB+cosAsinB