1/(x-3)^2 × (x-3)/(x+3) × (3-x)(3+x)
=1/[(x-3) × (x+3)] × (3-x)(3+x)
=1/[(x-3) × (x+3)] × (3-x)(x+3)
=1/(x-3) × (3-x)
=-1/(3-x)× (3-x)
=-1
（1-x）^2（3+x）^2/(x-y)^4(x-1)^2
=(x-1)^2（3+x）^2/(x-y)^4(x-1)^2
=（3+x）^2/(x-y)^4