原式=(3x^2+1/2y-2/3y^2)*(-1/8x^3y^3)
=3x^2*(-1/8x^3y^3)+1/2y*(-1/8x^3y^3)-2/3y^2*(-1/8x^3y^3)
=-3/8x^5y^3)-1/16x^3y^4+1/12x^3y^4