原式做以下变换：
x*x-4y*y-x+2y
=x*x-x+1/4-(4y*y-2y+1/4)
=(x-1/2)(x-1/2)-(2y-1/2)(2y-1/2)
利用平方差公式：
=(x-1/2+2y-1/2)(x-1/2-2y+1/2)
=(x+2y-1)(x-2y)