This applet shows the surface defined by [math]f(x,y)=4-0.5\sqrt{3x^2-2.5xy^2+y^4}[/math] along with the gradient vector [b][math]\nabla f\left(x_0,y_0\right)[/math][/b] at the point [math]P_2=(x_0,y_0)[/math]. The point [math]P_2[/math] can be dragged.