Areas and limits
Just as the general nation of a tangent line leads to the concept of limit, so does the general notion of area. For plane regions with straight-line boundaries, areas can often be calculated by subdividing the region into rectangles or triangles and adding the areas of the constituent parts (Figure2.1.6).However, for regions with curved boundaries, such as that in figure 2.1.7a,a more general approach is needed. One such approach is to being by approximating the area of the region by inscribing a number of rectangles of equal width under the curve and adding the areas of these rectangles (figure 2.1.7b). Intuition suggest that if we repeat that approximation process using more and more rectangles, then the rectangles will tend to fill in the gaps under the curve, and the approximations will get closer and closer to the exact area under the curve(Fiture2.1.7c). This suggests that we can define the area under the curve to be limiting value of these approximations. This idea will be considered in detail later, but the point to note here is that once again the concept of a limit comes into play.