1/(a+1)-(a+3)/(a^2-1)*(a^2-2a+1)/a^2+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)]/(a+1)^2
=2/(a+1)^2.=2/(a^2+2a+1)
因为a^2+2a-1=0
所以a^2+2a=1
原式=2/(1+1)=1