在△ABC中,证明(a2-b2)/c2=[sin(A-B)]/sinC.
由余弦定理,a2=b2+c2-2bccosA ① b2=a2+c2-2accosB ② ①-②得a2-b2=b2-a2-2bccosA+2accosB 整理得(a2-b2)/c2=(acosB-bcosA)/c 依正弦定理,有a/c=sinA/sinC,b/c=sinB/sinC 所以=(a2-b2)/c2=(sinAcosB-sinBcosA)/sinC=[sin(A-B)]/sinC 所以(a2-b2)/c2=[sin(A-B)]/sinC.