原式=a/(a+1)-(a-2)/[(a+1)(a-1)]×(a-1)²/[a(a-2)]
=a/(a+1)-(a-1)/[a(a+1)]
=(a²-a+1)/[a(a+1)] ,a≠1,-1,0,2
a=-2
原式=(4+2+1)/[(-2)×(-1)]=7/2