sina+sinb=2sin（1/2a+1/2b）cos（1/2a-1/2b）怎么来的啊?

sina+sinb=2sin（1/2a+1/2b）cos（1/2a-1/2b）怎么来的啊?
sina+sinb=2sin（1/2a+1/2b）cos（1/2a-1/2b）怎么来的啊?

sina+sinb=2sin（1/2a+1/2b）cos（1/2a-1/2b）怎么来的啊?
sina+sinb
=sin[(1/2a+1/2b)+(1/2a-1/2b)]+sin[(1/2a+1/2b)-(1/2a-1/2b)]
=sin(1/2a+1/2b)cos(1/2a-1/2b)+cos(1/2a+1/2b)sin(1/2a-1/2b)+sin(1/2a+1/2b)cos(1/2a-1/2b)-cos(1/2a+1/2b)sin(1/2a-1/2b)
=2sin(1/2a+1/2b)cos(1/2a-1/2b)
即
sina+sinb=2sin（1/2a+1/2b）cos（1/2a-1/2b）
得证