2sin(a+B)乘 cosa-sin(2a+B)-sinB
=2sin(a+B)乘 cosa-sin(a+a+B)-sinB
=2sin(a+B)乘 cosa-sinacos(a+B)-cosasin(a+B)-sinB
=sin(a+B)乘 cosa-sinacos(a+B)-sinB
=sin((a+B)-a)-sinB
=sin(B)-sinB
=0
=-2SINB
2sin(a+B)乘 cosa-sin(2a+B)-sinB
=2sin(a+B)乘 cosa-sin(a+a+B)-sinB
=2sin(a+B)乘 cosa-sinacos(a+B)-cosasin(a+B)-sinB
=sin(a+B)乘 cosa-sinacos(a+B)-sinB
=sin((a+B)-a)-sinB
=sin(B)-sinB
=0
=-2SINB