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+3)(a+1)
=1/(a+1)-(a-1)/(a+1)^2
=(a+1)/(a+1)^2-(a-1)/(a+1)^2
=2/(a^2+2a+1)
=2/2=1