>>753
3*x(x+2*y)dx+(3x^2-y^2)dy = d ((x^3)+3(x^2)y -(1/3)(y^3))
だから

∫[(0,0)〜(a,b)](3*x(x+2*y)dx+(3x^2-y^2)dy)
=((a^3)+3(a^2)b -(1/3)(b^3))