X1/(X2+1)+X2/(X1+1)
=[x1(x1+1)+x2(x2+1)] / (X1+1)(x2+1)
由韦达定理
x1+x2=1/3
x1x2=-1/3
x1^2+x2^2=1/9+2/3=5/9
所以原式=(5/9+1/3)/(-1/3+1/3+1)
=8/9