The demonstration builds upon the idea of x- and y-slice curves that was introduced in the previous lab. Choose a point (x₀, y₀) using the white hotspot in the domain window. The graph window shows the point (x₀,y₀,f(x₀,y₀) on the surface and the x- and y-slice curves passing through it. The other two windows show the tangent lines to the x- and y-slice curves at (x₀,y₀,f(x₀,y₀)). The slopes of these tangent lines are the partial derivatives f_x and f_y evaluated at (x₀,y₀).