X^2-3XY+2Y^2=(x-2y)(x-y) （十字相乘法）
或X^2-3XY+2Y^2
=X^2-3XY+2.25Y^2-0.25y^2
=(x-1.5y)^2-0.25y^2
=(x-1.5y+0.5y) (x-1.5y-0.5y)
=(x-y) (x-2y)