利用正弦定理：a/sinA=b/sinB=c/sinC,2bcosA=ccosA＋acosC>>>>> A=60° ===>>> cosA=[b²＋c²－a²]/(2bc)=[(b＋c)²－a²－2bc]/(2bc) ===>>>> (b＋c)²－a²－2bc=bc ====>>>> 3bc=1...