9(x-y)^2-30(x-y)+25 因式分解
问题描述：
9(x-y)^2-30(x-y)+25 因式分解
答
令x-y=t,
则该式为9t*t-30t+25=3t*3t-2*3t*5+5*5=(3t-5)(3t-5)
把t=x-y代入即得9（x-y）^2-30(x-y)+25=(3x-3y-5)^2.