This says that ∂f/∂x(a,b) gives the instantaneous rate of change of f with respect to x (i.e., in the x-direction) at the point (a,b). Graphically, observe that in defining ∂f/∂x(a,b), we are looking only at point in the plane y=b . The intersection of z=f(x,y) and y=b is a curve, as shown in Figures 12.20a and 12.20b (on the following page). The partial derivative ∂f/∂x(a,b) then gives the slope of the tangent line to this curve at x=a,as indicated in Figure 12.20b.