x^3+x^2*y+x*y^2-5=0
3x^2+(2xy+x^2*dy/dx)+(y^2+2xy*dy/dx)=0
所以(x^2+2xy）dy/dx=-(3x^2+2xy+y^2)
dy=-(3x^2+2xy+y^2)/(x^2+2xy)*dx