Using single variable calculus, we would search among points where the tangent line is horizontal for our minimum value. Here, we have a function of two variables, so we require a horizontal tangent plane to our surface. To find such a plane, it is necessary for both partial derivatives to be zero. With the sliders, locate points where both partial derivatives (dR/dt and dR/dp) are zero. How can you tell which of these points are local minima?