4X^2+2XY-2Y^2+4X+7Y-3 =4X^2+(2XY+4X)-(2Y^2-7Y+3) =4X^2+(2Y+4)X-(2Y-1)(Y-3)(十字相乘) 2X 2Y-1 2X -(Y-3) =(2X+2Y-1)(2X-Y+3)