9m^2-n^2+3m-n
问题描述：
9m^2-n^2+3m-n
4x^2-y^2-2y-1
答
9m^2-n^2+3m-n
＝(3m-n)*(3m+n)+(3m-n)
=(3m-n)*(3m+n+1)
4x^2-y^2-2y-1
=4x^2-(y^2+2y+1)
=4x^2-(y+1)^2
=(2x-y-1)*(2x+y+1)