4(m+n)的二次方-9(2m-n)的二次方
=[2(m+n)]²-[3(2m-n)]²
=(2m+2n+6m-3n)(2m+2n-6m+3n)
=(8m-n)(-4m+5n)
=(8m-n)(5n-4m)

9(x+y)的二次方-6(x+y)+1
=[3(x+y)]²-2×3(x+y)×1+1²
=(3x+3y-1)²