∵A+B=π-C
把此代入下式右边
π-C）sin(A-B)
Csin(A-B)
把右边移到左边，提取公因式即得
sin(A一B)(sin²A+sin²B-sin²C)=sin(A--B)(sin²A+sin²B)-sin(A一B)sin²C=sin(A一B)sin²(A+B)一Sin(A一B)Sin²C=sin(A一B)sin²(π-C)一Sin(A一B)Sin²C=0
