(a+b+c)(a+b-c)=3ab -> a^2+b^2-ab-c^2=0 -> c^2=a^2+b^2-ab
根据余弦定理得 c^2=a^2+b^2-2abcosC=a^2+b^2-ab -> -2abcosC=-ab -> cosC=1/2
因为C∈(0,180)-> C=60
sinC=2sinBcosA -> sin(A+B)=2sinBcosA -> sinAcosB+sinBcosA=2sinBcosA ->
sinAcosB-sinBcosA=0 -> sin(A-B)=0 -> A-B=0 -> A=B
又因为A+B=180-C=120 -> A=B=60
所以三角形为等边三角形