解原式=1/(x-1)-(x+3)/((x+1)(x-1))*[((x-1)(x-1))/((x+3)(x-1))]
=1/(x-1)-1/(x+1)
=2/(x^2-1)
当x=-1/3
原式=2/(1/9-1)
=2/(-8/9)
=-9/4