余弦定理：cosA = ( b^+c^-a^ )/ 2bc = (c-b)/2bcosB = ( a^+c^-b^ )/ 2ac = (b+c)/2acos2B = 2(cosB)^-1 = (c-b)/2b也即：cosA = cos2B又 a^=b^+ bc ,所以a大于b 2B小于180度A和2B都小于180度,只能是A=2B...