=[(a²+a-a)/(a+1)]÷[a(a-2)/(a+2)(a-2)]×[(a+1)/(a+1)(a+2)]
=a²/(a+1)÷a/(a+2)×1/(a+2)
=a²/(a+1)×(a+2)/a×1/(a+2)
=a/(a+1)