x3y3-1/2x2y+y2-2x3y3+0.5x2y+y2+x3y3+2y-3
=(x3y3-2x3y3+x3y3)+(-1/2x2y+0.5x2y)+(y2+y2)+2y-3
=0+0+2y2+2y-3
=2y^2+2y-3 值与字母x的取值无关
x3y3-1/2x2y+y2-2x3y3+0.5x2y+y2+x3y3+2y-3
=(x3y3-2x3y3+x3y3)+(-1/2x2y+0.5x2y)+(y2+y2)+2y-3
=0+0+2y2+2y-3
=2y^2+2y-3 值与字母x的取值无关