tanx-sinx=sinx/cosx-sinx =[sinx(1-cosx)]/cosx (tanx-sinx)/sin3x=(1-cosx/cosx)sin2x =(1-cosx/cosx)/1-cos2x =(1-cosx/cosx)/[(1-cosx)(1+cosx)] =1/[cosx(1+cosx)] lim趋向于0时应该=1/2
