题目a=2bcosc写错了吧,是a=2bcosC才对.因为a,b,c 成等比数列,所以有b^2=ac,根据余弦定理cosC=(a^2+b^2-c^2)/(2ab),则a=2bcosC=a=2b(a^2+b^2-c^2)/(2ab)=(a^2+b^2-c^2)/a,化简得b^2=c^2,所以b=c,代入b^2=ac得b=a.即a=...