ab=1,a-b=2,
a^2+b^2=(a-b)^2+2ab=4+2=6
b∕a＋a/b=(a^2+b^2)/ab=6/1=6
y=(x+3)^2 / (x+3)(x-3) * x(x-3) /(x+3) -x+3
=x-x+3
=3
所以不论x为任何有意义的值,y的值均不变,为3