结合律的作用 Proposition 3.19.Let G be a group.If a,bG,then (ab)-1=b-la-1. PRooF.Let a,bEG.Therabb-a-1=aca-1aa-1=e.Similarly,b-a-ab=e.But by the previous proposition,inverses are unique;hence,(ab)-1=b-la-1.结合律的作用