原式=(x+2y)(x-2y)+(x-2y)=(x-2y)(x+2y+1)
=（x-2y)(x+2y)+(x-2Y)=(x-2y)(x+2y+1)
x^2-4y^2+x-2y=(x+2y)(x-2y)+(x-2y)=(x-2y)(x+2y+1)
