Module 15  Riemann Sums and the Definite Integral  
Introduction  Lesson 1  Lesson 2  Lesson 3  SelfTest  
Lesson 15.2: Lefthand Riemann Sums and the area Program  
In Lesson 15.1 you used righthand rectangles to find the area of the region bounded by the graph of f(x) = x^{2}, the vertical line x = 1, and the xaxis. In this lesson you will use lefthand Riemann sums to find the same area. The sum of the areas of the rectangles shown above is called a lefthand Riemann sum because the lefthand corner of each rectangle is on the curve. Defining the Lefthand Sum Function A function similar to the one defined in Lesson 15.1 can be used to find the sum of the areas of the rectangles under the curve defined by f(x) between the vertical lines x = a and x = b and above the xaxis using lefthand rectangles. Modifying the command that defined rrs can create the new function.
The first lefthand xcoordinate is found by letting k = 0 and the last lefthand xcoordinate is found by letting k = n – 1. This replaces the first righthand xcoordinate found by letting k = 1 and the last righthand xcoordinate found by letting k = n.
The function f(x) = x^{2} should still be defined from Lesson 15.1. The following exploration uses the same procedure used to find the righthand Riemann sum using four rectangles, except this time the lefthand function lrs will be used to approximate the area under the curve.
The areas found using lefthand rectangles appear to converge to the same limit as the areas found using righthand rectangles, .
15.2.1 Evaluate limit(lrs(0,1,n),n, ) and interpret the result. Click here for the answer. When the number of rectangles approaches infinity, the lefthand function lrs has the same limit as the righthand function rrs. That is, the area under the curve can be found by using either right or left Reimann sum limits as the number of rectangles approaches because they produce the same result. This is true in general for continuous functions, and in fact, any xvalue in each subinterval on the xaxis can be used to determine the height of the corresponding rectangle. The area Program
A program can be used to illustrate the rectangles that approximate the area under a curve. The program area draws the rectangles associated with left, right and
Downloading the area Program
Transferring the Program to the TI89 Click here to get information about how to obtain the needed cable and to review the procedure to transfer the program from your computer to your calculator.
Using the area Program
The following procedure illustrates the approximate area of the region bounded by the graph of The function x^{2} should be stored in y1 in the Y= Editor.
The MAIN folder may be expanded by pressing , if necessary.
The parentheses should be empty. You should see a prompt for the value of a, the left side of the region.
Now you should see a prompt for the value of b, the right side of the region.
Next you see a prompt for the number of subintervals, i.e., the number of rectangles.
The graph of f(x) = x^{2} with the lefthand rectangles will be displayed when you press . The pause indicator should appear in the lower righthand corner of the screen. Viewing the Next Screens Subsequent screens of the program display the areas obtained by using all three methods and illustrations of the right and midpoint Riemann sums.
You should see the right Riemann sum, the illustration of midpoint rectangles, and finally the midpoint Riemann sum. The sums of the ten rectangles used to evaluate the area of the region using left, right, and midpoint rectangles are 0.285, 0.385, and 0.3325, respectively.
15.2.2 Use the area program to illustrate the area under f(x) = 2x + 1 between x = 0 and x = 3 using 6 rectangles, and find the approximate area using left, right, and midpoint rectangles. Click here for the answer. 

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