Understanding the Method of Rectangles

Oct 13, 2025 by wbrum.org 53 views

Introduction

In mathematics, definite integrals are used to calculate the area under curves and are a fundamental concept in calculus. However, in many cases, it is not possible to find the exact value of a definite integral. This is where approximation methods come into play. In this article, we will discuss how to approximate a definite integral using the method of rectangles.

The Problem

We are given the definite integral:

I=\int_{-5}^{+5} \frac{d x}{(x-2)^2}

with a step size of $h=2$ . Our goal is to find an approximation for this integral.

Understanding the Method of Rectangles

The method of rectangles is a simple and effective way to approximate a definite integral. The basic idea is to divide the area under the curve into small rectangles and then sum up the areas of these rectangles. The height of each rectangle is determined by the value of the function at the midpoint of the interval.

Step 1: Divide the Interval into Subintervals

To apply the method of rectangles, we need to divide the interval $[-5, 5]$ into subintervals. Since the step size is $h=2$ , we will divide the interval into 3 subintervals:

[-5, -2], [-2, 2], [2, 5]

Step 2: Find the Midpoints of the Subintervals

Next, we need to find the midpoints of each subinterval:

x_1 = -\frac{3}{2}, x_2 = 0, x_3 = \frac{7}{2}

Step 3: Evaluate the Function at the Midpoints

Now, we need to evaluate the function at each midpoint:

f(x_1) = \frac{1}{(-\frac{3}{2}-2)^2} = \frac{1}{\frac{49}{4}} = \frac{4}{49}

f(x_2) = \frac{1}{(0-2)^2} = \frac{1}{4}

f(x_3) = \frac{1}{(\frac{7}{2}-2)^2} = \frac{1}{\frac{9}{4}} = \frac{4}{9}

Step 4: Calculate the Area of Each Rectangle

The area of each rectangle is given by the product of the height and the width. Since the width of each rectangle is $h=2$ , the area of each rectangle is:

A_1 = f(x_1) \cdot h = \frac{4}{49} \cdot 2 = \frac{8}{49}

A_2 = f(x_2) \cdot h = \frac{1}{4} \cdot 2 = \frac{1}{2}

A_3 = f(x_3) \cdot h = \frac{4}{9} \cdot 2 = \frac{8}{9}

Step 5: Sum Up the Areas of the Rectangles

Finally, we sum up the areas of the rectangles to get an approximation of the definite integral:

I \approx A_1 + A_2 + A_3 = \frac{8}{49} + \frac{1}{2} + \frac{8}{9}

Simplifying the Expression

To simplify the expression, we need to find a common denominator. The least common multiple of 49, 2, and 9 is 441. Therefore, we can rewrite the expression as:

I \approx \frac{8 \cdot 9}{49 \cdot 9} + \frac{1 \cdot 441}{2 \cdot 441} + \frac{8 \cdot 49}{9 \cdot 49}

I \approx \frac{72}{441} + \frac{441}{882} + \frac{392}{441}

I \approx \frac{72}{441} + \frac{392}{441} + \frac{441}{882}

I \approx \frac{464}{441} + \frac{441}{882}

I \approx \frac{464}{441} + \frac{147}{294}

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I \approx \frac{464}{441} + \frac{147}{294}

I \approx \frac{464}{441} + \frac{147}{294}<br/> **Frequently Asked Questions: Approximating Definite Integrals** ===========================================================

Q: What is the method of rectangles?

A: The method of rectangles is a simple and effective way to approximate a definite integral. The basic idea is to divide the area under the curve into small rectangles and then sum up the areas of these rectangles.

Q: How do I divide the interval into subintervals?

A: To apply the method of rectangles, you need to divide the interval into subintervals. The number of subintervals is determined by the step size, which is the distance between each subinterval.

Q: What is the midpoint of a subinterval?

A: The midpoint of a subinterval is the point that is equidistant from the two endpoints of the subinterval.

Q: How do I evaluate the function at the midpoints?

A: To evaluate the function at the midpoints, you need to plug the midpoint into the function and calculate the result.

Q: What is the area of each rectangle?

A: The area of each rectangle is given by the product of the height and the width. The height of each rectangle is determined by the value of the function at the midpoint, and the width is the step size.

Q: How do I sum up the areas of the rectangles?

A: To sum up the areas of the rectangles, you need to add up the areas of each rectangle.

Q: What is the final answer?

A: The final answer is the sum of the areas of the rectangles.

Q: What are some common mistakes to avoid?

A: Some common mistakes to avoid when approximating definite integrals using the method of rectangles include:

Not dividing the interval into enough subintervals
Not evaluating the function at the midpoints correctly
Not calculating the area of each rectangle correctly
Not summing up the areas of the rectangles correctly

Q: What are some real-world applications of approximating definite integrals?

A: Approximating definite integrals has many real-world applications, including:

Calculating the area under curves in physics and engineering
Calculating the volume of solids in geometry
Calculating the probability of events in statistics
Calculating the work done by a force in physics

Q: What are some other methods for approximating definite integrals?

A: Some other methods for approximating definite integrals include:

The method of trapezoids
The method of Simpson's rule
The method of Gaussian quadrature
The method of Romberg's method

Q: How do I choose the best method for approximating a definite integral?

A: The best method for approximating a definite integral depends on the specific problem and the desired level of accuracy. You should choose a method that is suitable for the problem and that will give you the desired level of accuracy.

Q: What are some tips for improving the accuracy of approximations?

A: Some tips for improving the accuracy of approximations include:

Using a smaller step size
Using a larger number of subintervals
Using a more accurate method
Checking the results for errors

Q: What are some common errors to look out for?

A: Some common errors to look out for when approximating definite integrals include:

Rounding errors
Truncation errors
Approximation errors
Calculation errors

Q: How do I check the accuracy of my approximations?

A: To check the accuracy of your approximations, you should:

Compare your results with the exact value of the integral
Check for errors in your calculations
Use a more accurate method to verify your results
Check for consistency in your results

Q: What are some resources for learning more about approximating definite integrals?

A: Some resources for learning more about approximating definite integrals include:

Textbooks on calculus and numerical analysis
Online tutorials and videos
Research papers and articles
Online forums and communities

Q: How do I apply approximating definite integrals in real-world problems?

A: To apply approximating definite integrals in real-world problems, you should:

Identify the problem and the desired level of accuracy
Choose a suitable method for approximating the definite integral
Apply the method to the problem
Check the accuracy of the results
Use the results to solve the problem.