晕,数学书上讲cos(a+b)=cosacoab-sinasinb
sin(a+b)=cos(π/2-a-b)=cos(π/2-a)cosb-sin(π/2-a)sin(-b)=sinacosb+cosasinb
把b=-b代入上式得
sin(a-b)=sinacosb-cosasinb