for any values of x for which the limit exists. At any particular value x=a, we interpret (a) as the instantaneous rate of change of the funtion with respect to x at that point. Consider a flat metal plate in the shape of the region . Suppose that the temperature at any point (x,y) is given by . If you move along the horizontal line segment form(a,b)to (a+h,b),whay is the average rate of change of the temperature with repect to the horizontal distance x (see Frigure 12.19)? Notice that on this line segment, y is a constant (y=b). So, the average rate of change on this line segment is given by