原式=1/(x+1)-(x+3)/(x+1)(x-1)×(x-1)²/(x+1)(x+3)
=(x+1)/(x+1)²-(x-1)/(x+1)²
=(x+1-x+1)/(x+1)²
=2/(x+1)²
=2/(-2+1)²
=2