x/x+1-x+3/x-1*x的二次方-2x+1/x的二次方+4x+3
=x/(x+1)+(x+3)/(x-1)×(x-1)²/(x+1)(x+3)
=x/(x+1)+(x-1)/(x+1)
=(x+x-1)/(x+1)
=(2x-1)/(x+1)
当x=2/3时
原式=(1/3)/(5/3)=1/5