SIN(B+C+B-C)=SIN2B=SIN(B+C）COS(B-C)+COS(B+C)SIN(B-C)''''''''''''''''''''''1式
SIN(B+C-B+C)=SIN2C=SIN(B+C)COS(B-C)-COS(B+C)SIN(B-C)'''''''''''''''''''''''2式
1式加2式就得到了