In the previous video, we graphed curves onto 3D and we worked with an equation that represented the curve in between a constant sum and a constant product. The equation was x times y divided by d over 2 squared. We are going to take a closer look at this equation in 3D. When we graph this equation, we get a 3D surface. ```javascript f(x,y)=xy/((d/2)^2) ``` The inputs are all the points on the x y plane and the output is described in the z-axis. The result is a 2D surface in 3D. Now, we want to know how this function behaves on the points of the curves V1 curve. First, we will plot the equation x times y divided by d over 2 squared for all of the points on the constant product. We will take the x and y points that satisfy x times y equals d over 2 squared and then we will evaluate the equation. The result is a blue line on the 2D surface. ```javascript f(x,y)=xy/((d/2)^2) ``` The z-axis value is always 1 for this blue line. This is because x times y is always equal to d over 2 squared. This function will always evaluate to 1. Now, we will look at the points on the constant sum curve. This produces an arc on the surface. The arc touches 1 and decreases to 0 as y goes to 0, and also decreases to 0 as x goes to 0. We can plot a point at (d over 2, d over 2, 1) to see where this equation evaluates to 1. We can also see that this orange line approaches 0 as x and y become more unbalanced. When we evaluate this equation for the curves V1 curve, we get the orange line. The line represents the z-axis value for each point of the V1 curve. The orange line approaches 0 as x and y become more unbalanced. We have now analyzed how x times y divided by d over 2 squared behaves in 3D. When x and y are close to d over 2, then the equation evaluates to 1. As x and y become more unbalanced, they approach 0.