3x³-2xy²+4x²y+xy²-4x²y
合并同类项得3x³-2xy²+xy²+4x²y-4x²y
得3x³-xy²
由题意知x=1/3,y=-2
带回原式得 3*1/3^3-1/3*（-2）^2
=1/9-1/3*4
=1/9-4/3
=1/9-12/9
=-11/9