原式=[(a+1)²-5-(a-1)(a+3)]/2(a+1)(a-1)×4(a-1)/3=(a²+2a+1-5-a²-2a+3)/2(a+1)(a-1)×4(a-1)/3=-1/2(a+1)(a-1)×4(a-1)/3=-2/(3a+3)=-2/(-3/2+3)=-4/3