令k=a/sinA=b/sinB=c/sinC所以a=ksinAb=ksinB c=ksinC代入acosA+bcosB=ccosC,并约去ksinAcosA+sinBcosB=sinCcosCsin2A+sin2B=2sinCcosCsin[(A+B)+sin(A-B)]+sin[(A+B)-sin(A-B)]=2sinCcosCsin(A+B)cos(A-B)+cos(A+B)...