x的平方+2x+1/2x-6/(x-1-3x/x-3),其中x=5=(x+1)^2/2x-6/{[(x-1)(x-3)-3x]/(x-3)},=(x+1)^2/2(x-3) * (x-3)/(x^2-4x+3-3x)=(x+1)^2/2(x^2-7x+3)=(5+1)^2/2(5^2-7*5+3)=36/(-14)=-18/7,