sin(60-50)+sin50-sin70
=sin60cos50-cos60sin50+sin50-sin70
=sin(60+50)-sin70
=sin70-sin70=0=sin(60+50)-sin70这步不会看。sin(110)=sin(180-70)=sin70=sin60cos50-cos60sin50+sin50-sin70这步到下一步的.=sin(60+50)-sin70sin60cos50-(1/2)sin50+sin50-sin70=sin60cos50+(1/2)sin50-sin70=sin60cos50+cos60sin50-sin70=sin(110)-sin70