a²+2a-8=0
(a-2)(a+4)=0
a-2=0 或 a+4=0
a=2 或 a=-4
原式=1/(a+1)-[(a+3)/(a²-1)]×[(a²-2a+1)/(a²+4a+3)]
=1/(a+1)-{ (a+3)/[(a+1)(a-1)] }×{ (a-1)²/[(a+1)(a+3)] }
=1/(a+1)-(a-1)/(a+1)²
=(a+1)/(a+1)²-(a-1)/(a+1)²
=(a+1-a+1)/(a+1)²
=2/(a+1)²
当a=2时：
原式=2/(2+1)²
=2/3²
=2/9
当a=-4时：
原式=2/(-4+1)²
=2/(-3)²
=2/9