9（x+2y）的平方-4（2x+y）的平方
=[3(x+2y)]²-[2(2x+y)]²
=(3x+6y+4x+2y)(3x+6y-4x-2y)
=(7x+8y)(-x+4y)
=-(7x+8y)(x-4y)
4（x-2y)的平-4x（x-2y)+x的平方
=[2(x-2y)-x]²
=(x-4y)²