方法1：
X+(XY-4X-3Y)
=x+(xy-4x-4y+y)
=x+[xy-4(x+y)+y]
=x+3-1+y
=2+1/4
=4/9
方法2：
x+(xy-4x-3y)=x+xy-4x-3y=xy-3x-3y=xy-3(x+y)=3-3*1/4=9/4