过点A作AE⊥BC于E,
∵AB = AC,且∠BAC = 120°,
∴∠BAC = ∠EAC = 60°,
∴BE = sin60°*AB = 根号3AB/2
同理:EC = 根号3AB/2
∴BC = 根号3AB
∴AB:BC = 根号3/3