原式=(x+1)//(x²+1)*(x²+1)(x+1)(x-1)/(x+1)³-(x-3)/(x+1)
=(x-1)/(x+1)-(x-3)/(x+1)
=(x-1-x+3)/(x+1)
=-2/(x+1)
=-2/(3+1)
=-1/2