y=(x^2+6x+9)/(x^2-9) ÷（x+3）/(x^-3x)-x+3
y=(x+3)²/(x-3)(x+3)÷(x+3)/x(x-3)-x+3
y=(x+3)/(x-3)×x(x-3)/(x+3)-x+3
y=x-x+3
y=3
∴ y=(x^2+6xs+9)/(x^2-9) ÷（x+3）/(x^-3x)-x+3,试说明不论x为任何有意义的值,y的值均不变.