Calculate the expression of the antiderivative

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Discussion Overview

The discussion revolves around the calculation of antiderivatives, specifically whether it is possible to derive the expression of an antiderivative directly from the integrand without relying on established tables of functions and their antiderivatives. The scope includes theoretical aspects of integration and practical examples.

Discussion Character

  • Exploratory
  • Technical explanation
  • Conceptual clarification
  • Debate/contested
  • Mathematical reasoning

Main Points Raised

  • Some participants suggest that while one can express the antiderivative as a limit of Riemann sums, this does not yield a closed expression without prior knowledge of the result.
  • It is mentioned that if \( \frac{dF(x)}{dx} = f(x) \), then \( F(x) - F(0) = \int_0^x f(u) du \), but it is questioned whether this aligns with the original inquiry.
  • One participant seeks a formula to calculate the antiderivative from the integrand, using \( x^2 \) as an example, and notes that while there is a formula for polynomials, it does not extend to all functions.
  • Another participant points out that the derivative of \( x^n \) can be used to derive the antiderivative \( F(x) = \frac{1}{n+1}x^{n+1} \), but emphasizes that a general formula is not available.
  • Concerns are raised about the necessity of memorizing derivatives and the conditions under which the derivative expression is valid, particularly regarding the limit process.

Areas of Agreement / Disagreement

Participants express differing views on the ability to derive antiderivatives from integrands. While some agree on specific cases, such as polynomials, others argue that a general formula does not exist, indicating ongoing uncertainty and debate.

Contextual Notes

Limitations include the dependence on specific types of functions for which formulas exist, and the unresolved nature of applying these concepts to more complex integrands.

hugo_faurand
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Hello everyone !
I've started to work on integral and I wonder if it's possible to calculate the expression of the antiderivative with the expression of the "integrand"1 rather than use a table with the function and its antiderivative.

Thank you in advance !

1( I'm french and I d'ont know the translation for this word, so I make an assumption and I put quotes)
 
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hugo_faurand said:
Hello everyone !
I've started to work on integral and I wonder if it's possible to calculate the expression of the antiderivative with the expression of the "integrand"1 rather than use a table with the function and its antiderivative.

Thank you in advance !

1( I'm french and I d'ont know the translation for this word, so I make an assumption and I put quotes)
On can write the antiderivative as a limit of Riemannian sums, which are expressions of the integrand1, but this doesn't give you a closed expression. This can only eventually be done after you know the result, as for ##\int \sin x dx = -\cos x = \sin (x-\frac{\pi}{2})## which is more by chance than by an actual dependency.

1) Neither am I, so this might still be the wrong word.
 
Yes. If ##\ {dF(x)\over dx} = f(x)\ ## then ##F(x) - F(0) = \int_0^x f(u) du \ ##, and ##F(x)## is an antiderivative of ##f(x)##. But is this what you meant to ask ?
 
BvU said:
Yes. If ##\ {dF(x)\over dx} = f(x)\ ## then ##F(x) - F(0) = \int_0^x f(u) du \ ##, and ##F(x)## is an antiderivative of ##f(x)##. But is this what you meant to ask ?

In fact, if I have the expression of the integrand, Can I calculate the antiderivative ?
For example, I search the antiderivative of x². I would like to know if it exists a kind of formula to calculate the antiderivative with the expression of the integrand.
 
You know that ##\ {d\over dx} x^n = nx^{n-1},\ ## so if ##\ F(x) = {1\over n+1}x^{n+1},\ ## then F(x) is an antiderivative of ##x^n##.

This satisfies the criterion 'a formula to calculate the antiderivative' for a specific kind of function ##f##.
A general recipe is not available, so it remains a kind of 'metier', or better: 'artisanat'
 
hugo_faurand said:
In fact, if I have the expression of the integrand, Can I calculate the antiderivative ?
For example, I search the antiderivative of x². I would like to know if it exists a kind of formula to calculate the antiderivative with the expression of the integrand.
In this case, it is the formula ##x^n \longmapsto \dfrac{1}{n+1}x^{n+1}## but this only covers polynomials, and in general, the answer is no. You cannot write down straight away a formula for, say ##\int{\dfrac{\cot (1+x^2)}{\tan (1-x^2)}\, dx}##.
 
So, I just have to learn by heart my derivative.
Another little question can I use this expression to calculate all the derivatives I want or Is there exeptions ?

$$ f'(x) = \lim_{dx\to 0} \frac{f(x+dx)-f(x)}{dx}$$
 
Last edited:
Do not forget to take the limit ##dx\rightarrow 0## !
If the limit does not exist, then the derivative also does not exist !
 
  • #10
BvU said:
Do not forget to take the limit ##dx\rightarrow 0## !
If the limit does not exist, then the derivative also does not exist !
Yes, corrected !
 

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