sin^2A-cos^2A=1/2cos2A=-1/22A=120°A=60°所以b2+c2-a2=bcb2+c2+2bc=a2+3bc(b+c)^2=a2+3bc假如b+c=2a则a2=bcb2+c2=2bc仅扰亏含当b=c成缓笑立空孝