-ab^2+2a^2b-a^3b
=-ab(1-2a+a^2)
=-ab(1-a)^216m^4-8m^2n^2+n^4 再问一下16m^4-8m^2n^2+n^4 =(4m^2-n^2)^2=(2m+n)^2(2m-n)^2