1.原式=（x-y）^2+2*2*(x-y)+2^2=(x-y+2)^2
2.原式=2（x^2-2x+2^2）=2(x-2)^2
3.原式=4m^2n(2m-n)-n^3（n-2m)=n(n-2m)(4m^2-n^2)=n(n-2m)(2m-n)(2m+n)
4.原式=-[x^2-2*2xy+(2y)^2]=-(x-2y)^2