原式=[1/(x+1)]-[(x+3)/(x²-1)]×[(x²-2x+1)/(x²+4x+3)]
=[1/(x+1)]-{ (x+3)/[(x+1)(x-1)] }×{ (x-1)²/[(x+1)(x+3)] }
=[1/(x+1)]-[(x-1)/(x+1)²]
=[(x+1)/(x+1)²]-[(x-1)/(x+1)²]
=[(x+1)-(x-1)]/(x+1)²
=(x+1-x+1)/(x+1)²
=2/(x+1)²