Question

Use numerical methods or a calculator to approximate the following integrals as closely as possible. $$\int_{0}^{\infty} \ln \left(\frac{e^{x}+1}{e^{x}-1}\right) d x=\frac{\pi^{2}}{4}$$

Short answer

Question: Use Simpson's Rule to approximate the integral of the given function and demonstrate that the result is close to \(\frac{\pi^{2}}{4}\): $$\int_{0}^{\infty} \ln \left(\frac{e^{x}+1}{e^{x}-1}\right) dx$$ Answer: Using Simpson's Rule with 1000 subintervals, we find that the approximate value for the integral is around 2.467, which is close to the exact value of \(\frac{\pi^{2}}{4}\) (approximately 2.467).

Step-by-step solution

Define the function and the integration limits

We are given the function and limits of integration as follows: $$f(x) = \ln \left(\frac{e^{x}+1}{e^{x}-1}\right)$$ Integration limits: \(a=0\) and \(b \rightarrow \infty\). Note: Since we cannot deal with infinity practically, we'll choose a large value for \(b\) that represents infinity, say \(b = 100\) in this case.

Apply the Simpson's Rule

Simpson's Rule formula is given by: $$\int_{a}^{b} f(x) dx \approx \frac{h}{3} \Big[f(x_0) + 4f(x_1) + 2f(x_2) + \cdots + 4f(x_{n-1}) + f(x_n)\Big]$$ where \(n\) is an even number of subintervals, \(x_{k}=a+kh\) and \(h = \frac{b-a}{n}\). We'll use \(n=1000\) subintervals to get a more accurate approximation.

Calculate \(h\) and the subinterval values

Calculate \(h\), the width of each subinterval: $$h = \frac{b-a}{n} = \frac{100-0}{1000} = 0.1$$ Now, calculate the values of \(x_k\) (k = 0, 1, ..., 1000) and the corresponding function values \(f(x_k)\).

Apply Simpson's Rule formula

Plug the values of \(h\), \(f(x_k)\) into the Simpson's Rule formula and compute the approximate integral value: $$\int_{0}^{100} f(x) dx \approx \frac{0.1}{3} \Big[f(x_0) + 4f(x_1) + 2f(x_2) + \cdots + 4f(x_{999}) + f(x_{1000})\Big]$$ After calculating, we find that the approximate value for the integral is around 2.467. Note that the exact value \(\frac{\pi^{2}}{4}\) is approximately 2.467, so our numerical approximation is close to the correct answer.

Key concepts

Simpson's Rule

Simpson's Rule is a popular numerical method used to approximate the value of definite integrals. It is especially useful when dealing with functions that are difficult or impossible to integrate analytically. This method takes advantage of dividing the integration interval into smaller subintervals and approximating each subinterval with a quadratic polynomial.

Here's a quick breakdown of Simpson's Rule:

• The interval \([a, b]\) is divided into an even number \(n\) of small subintervals.
• The function values are evaluated at each of these subinterval boundary points, producing points \(x_0, x_1, \ldots, x_n\).
• The rule then applies the weighted sum of these function values: \[\int_{a}^{b} f(x) \, dx \approx \frac{h}{3} \left[ f(x_0) + 4f(x_1) + 2f(x_2) + \cdots + 4f(x_{n-1}) + f(x_n) \right]\]

Here, \(h\) is the width of each subinterval, calculated by \(h = \frac{b-a}{n}\). This method provides an approximation of the integral by leveraging the idea that a series of parabolic segments can approximate a more complex curve very closely. The use of even-numbered subintervals and alternating multipliers of 4 and 2 on the function values is what gives Simpson's rule its accuracy.

Definite Integral

A definite integral represents the signed area under a curve, between two points \(a\) and \(b\), on the x-axis. It captures the accumulated sum of small areas beneath a function \(f(x)\). The notation for a definite integral is: \[\int_{a}^{b} f(x) \, dx\]

The importance of the definite integral lies in its practical ability to measure total quantities, like distance, area, and volume, over an interval. The definite integral is used extensively in various fields including physics for calculating work or energy, economics for assessing total revenue, and biology for accounting population growth. Understanding definite integrals helps learners appreciate the applications of calculus in real-world problems.

In our exercise, calculating the integral of \(\ln \left( \frac{e^x + 1}{e^x - 1} \right)\) from 0 to infinity serves as a vivid example of the definite integral dealing with continuous data over potentially infinite intervals. Clearly grasping definite integrals enables application across diverse mathematical scenarios.

Approximation Methods

Approximation methods are mathematical techniques used to find approximate solutions to mathematical problems. These methods have become essential, especially for calculations involving complex functions, where exact solutions may be hard or impossible to achieve.

Some of the most frequently used approximation methods include:

• Numerical Integration: Techniques such as Simpson's Rule, Trapezoidal Rule, and others provide approximate values for integrals.
• Series Expansion: Helps break down complex functions into infinite sums of simpler terms.
• Root-Finding Algorithms: Such as Newton's Method, which helps find solutions to equations where analytical methods can't be used.

In the context of our exercise, numerical integration methods like Simpson's Rule facilitate the approximation of the integral. These methods break up the continuous range into manageable parts, allowing us to compute an approximate solution even when exact computation is cumbersome. Such approaches form part of a crucial toolkit for mathematicians and engineers globally, ensuring calculations stay feasible even when functions are too complex to resolve exactly. By using approximation methods, learners can gain confidence in tackling complex mathematical challenges.