余弦定理a^2-b^2=c^2-2bccosA a^2-b^2/c^2=(c-2bosA)/c 由正弦定理知a=2RsinA,B=2RsinB,C=2RsinC =(c-2bosA)/c=(sinC-2sinBcosA)/sinC=(sin(A+B)-2sinBcosA)/sinC=(sinAcosB-sinBcosA)/sinC =sin(A-B)/sinC...