The Formula and Its Conditions

Oct 13, 2025 by talynt.org 146 views

Introduction

In calculus, the definite integral of a function is a fundamental concept that represents the area under the curve of the function within a given interval. However, calculating the exact value of a definite integral can be challenging, especially for complex functions. To address this issue, mathematicians have developed various approximation methods, including the formula $\int_a^{a+h} f(x) \, dx \approx h f(a+h)-\frac{h^2}{2} f^{\prime}(a)$ . In this article, we will explore the conditions under which this formula is exact for polynomials of a certain maximum degree.

The Formula and Its Conditions

The given formula is a Taylor series approximation of the definite integral of a function $f(x)$ over the interval $[a, a+h]$ . The formula is based on the Taylor series expansion of the function $f(x)$ around the point $a$ , which is given by:

f(x) = f(a) + f^{\prime}(a)(x-a) + \frac{f^{\prime\prime}(a)}{2!}(x-a)^2 + \frac{f^{\prime\prime\prime}(a)}{3!}(x-a)^3 + \ldots

By integrating this series over the interval $[a, a+h]$ , we obtain:

\int_a^{a+h} f(x) \, dx = h f(a) + \frac{h^2}{2} f^{\prime}(a) + \frac{h^3}{3\cdot 2!} f^{\prime\prime}(a) + \frac{h^4}{4\cdot 3!} f^{\prime\prime\prime}(a) + \ldots

The given formula is an approximation of this series, truncated after the second term. This means that the formula is exact only if the remaining terms of the series are zero.

Polynomials of Maximum Degree

To determine the maximum degree of polynomials for which the formula is exact, we need to consider the terms that are omitted in the approximation. The first omitted term is of degree 3, which is given by $\frac{h^3}{3\cdot 2!} f^{\prime\prime}(a)$ . This term is zero if the function $f(x)$ is a polynomial of degree at most 2.

Proof

To prove that the formula is exact for polynomials of degree at most 2, we need to show that the remaining terms of the series are zero. Let $f(x)$ be a polynomial of degree at most 2, which can be written as:

f(x) = a_0 + a_1 x + a_2 x^2

where $a_0, a_1, a_2$ are constants. Then, the Taylor series expansion of $f(x)$ around the point $a$ is:

f(x) = a_0 + a_1 (x-a) + a_2 (x-a)^2

By integrating this series over the interval $[a, a+h]$ , we obtain:

\int_a^{a+h} f(x) \, dx = h a_0 + \frac{h^2}{2} a_1 + \frac{h^3}{3\cdot 2!} a_2

The given formula is an approximation of this series, truncated after the second term. This means that the formula is exact only if the remaining terms of the series are zero. Since $a_2 = 0$ for polynomials of degree at most 2, the remaining term is zero, and the formula is exact.

Conclusion

In conclusion, the formula $\int_a^{a+h} f(x) \, dx \approx h f(a+h)-\frac{h^2}{2} f^{\prime}(a)$ is exact for polynomials of degree at most 2. This means that the formula can be used to approximate the definite integral of a polynomial of degree at most 2 with an error of at most $O(h^3)$ . This result has important implications for numerical analysis and approximation theory, where the accuracy of numerical methods is crucial.

References

[1] Taylor, B. (1715). Methodus Incrementorum Directa et Inversa. London: J. Knapton.
[2] Euler, L. (1740). Institutiones Calculi Integralis. St. Petersburg: Academia Scientiarum.
[3] Cauchy, A. (1823). Cours d'Analyse. Paris: Bachelier.

Glossary

Taylor series: A power series representation of a function around a point.
Definite integral: The area under the curve of a function within a given interval.
Polynomial: A function of the form $a_n x^n + a_{n-1} x^{n-1} + \ldots + a_1 x + a_0$ .
Approximation theory: The study of methods for approximating functions and their properties.
Q&A: Approximating Definite Integrals with the Formula $\int_a^{a+h} f(x) \, dx \approx h f(a+h)-\frac{h^2}{2} f^{\prime}(a)$ ===========================================================

Q: What is the formula $\int_a^{a+h} f(x) \, dx \approx h f(a+h)-\frac{h^2}{2} f^{\prime}(a)$ used for?

A: The formula is used to approximate the definite integral of a function $f(x)$ over the interval $[a, a+h]$ . It is a Taylor series approximation of the integral, truncated after the second term.

Q: What are the conditions under which the formula is exact?

A: The formula is exact for polynomials of degree at most 2. This means that if the function $f(x)$ is a polynomial of degree at most 2, the formula will give the exact value of the definite integral.

Q: Why is the formula not exact for polynomials of degree greater than 2?

A: The formula is not exact for polynomials of degree greater than 2 because the remaining terms of the Taylor series are not zero. These terms are of degree 3 and higher, and they contribute to the error in the approximation.

Q: What is the error in the approximation?

A: The error in the approximation is of order $O(h^3)$ . This means that the error decreases cubically with the size of the interval $h$ .

Q: Can the formula be used to approximate the definite integral of a function that is not a polynomial?

A: Yes, the formula can be used to approximate the definite integral of a function that is not a polynomial. However, the error in the approximation will be larger than $O(h^3)$ , and the formula may not be as accurate.

Q: How can the formula be used in practice?

A: The formula can be used in practice to approximate the definite integral of a function in a variety of situations. For example, it can be used to estimate the area under a curve in a graphing calculator or computer program.

Q: What are some common applications of the formula?

A: Some common applications of the formula include:

Estimating the area under a curve in a graphing calculator or computer program
Approximating the definite integral of a function in a numerical analysis or approximation theory context
Solving problems in physics, engineering, or other fields that involve definite integrals

Q: Can the formula be used to approximate the definite integral of a function with a discontinuity?

A: No, the formula cannot be used to approximate the definite integral of a function with a discontinuity. The formula assumes that the function is continuous and differentiable over the interval $[a, a+h]$ , and it will not give accurate results if the function has a discontinuity.

Q: What are some common mistakes to avoid when using the formula?

A: Some common mistakes to avoid when using the formula include:

Not checking that the function is continuous and differentiable over the interval $[a, a+h]$
Not using a small enough value of $h$ to ensure that the error in the approximation is small
Not considering the possibility of a discontinuity in the function

Q: How can the formula be extended to approximate the definite integral of a function over a larger interval?

A: The formula can be extended to approximate the definite integral of a function over a larger interval by using a smaller value of $h$ and summing the results. This is known as the method of subintervals.

Q: What are some other methods for approximating the definite integral of a function?

A: Some other methods for approximating the definite integral of a function include:

The trapezoidal rule
Simpson's rule
The method of Gaussian quadrature
The method of Monte Carlo integration

Q: How can the formula be used in conjunction with other methods for approximating the definite integral of a function?

A: The formula can be used in conjunction with other methods for approximating the definite integral of a function by using it as a starting point and then refining the result using another method. This can be useful for improving the accuracy of the approximation.