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证明sin(a+b)-sina=2sinb/2*cos(a+b/2)
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证明sin(a+b)-sina=2sinb/2*cos(a+b/2)
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证明：sin(a+b)-sina=sin[(a+b/2)+b/2]-sin[(a+b/2)-b/2]
=sin(a+b/2)*cos(b/2)+cos(a+b/2)*sin(b/2)-[sin(a+b/2)*cos(b/2)-cos(a+b/2)*sin(b/2)]
=2sin(b/2)*cos(a+b/2)
=sin(a+b/2)*cos(b/2)+cos(a+b/2)*sin(b/2)-[sin(a+b/2)*cos(b/2)-cos(a+b/2)*sin(b/2)]
=2sin(b/2)*cos(a+b/2)
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