a^2+b^2=c^2+ab c^2=a^2+b^2-ab 余弦定理,c^2=a^2+b^2-2*a*b*cosC cosC=1/2,C=60°,A+B=120°cos(A+B)=cosAcosB-sinAsinB=-1/2,cosAcosB=1/4,cos(A-B)=cosAcosB+sinAsinB=1,A-B=0,可得A=B=C=60°,为等边三角形...