原式=(a²+a-a)/(a+1)×(a²-4)/(a²-2a)×1/(a+2)
=a²/(a+1)×(a+2)(a-2)/a(a-2)×1/(a+2)
=a²/(a+1)×(a+2)/a×1/(a+2)
=a(a+2)/(a+1)×1/(a+2)
=a/(a+1)