sin（a+b）cosa-1/2[sin(2a+b)-sinb]
=(sinacosb+cosasinb)cosa-1/2(sin2acosb+cos2asinb-sinb)
和差公式,全部打开
=sinacosacosb+cos²asinb-1/2(2sinacosacosb+(2cos²a-1)sinb-sinb)
二倍角公式打开
=sinacosacosb+cos²asinb-1/2(2sinacosacosb+2cos²asinb-sinb-sinb)
依次化简
=sinacosacosb+cos²asinb-sinacosacosb+cos²asinb-sinb
=-sin