Why can the dx in an integral be treated as a differential?

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Discussion Overview

The discussion revolves around the treatment of the "dx" in integrals, particularly in the context of substitution in calculus. Participants explore the conceptual understanding of "dx" as a differential and its implications in integral evaluation.

Discussion Character

  • Conceptual clarification, Technical explanation

Main Points Raised

  • One participant questions why "dx" can be treated as a differential in integrals, seeking insight into its conceptual basis.
  • Another participant provides an example involving differentiation and integration to illustrate the relationship between "dy" and "dx".
  • A different participant notes that without the integral sign, "dx" is considered lazy notation and not rigorously defined, suggesting that it conceals deeper results such as the Fundamental Theorems of Calculus.
  • It is mentioned that "dx" can be rigorously defined and is often used in advanced topics like Vector Calculus.

Areas of Agreement / Disagreement

Participants express varying levels of understanding regarding the treatment of "dx". While some provide examples and explanations, there is no consensus on a definitive answer to the initial question posed.

Contextual Notes

Some statements reflect a lack of rigor in the definition of "dx" in basic calculus, indicating that the discussion may depend on the context in which "dx" is used.

G01
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I was just wondering about the dx and the end of an integral and evaluating integrals by substitution. When you evaluate integrals by substitution you can treat the dx as the differential of x. This seems to convenient lol. Some one must ahve known that the dx in an integral was the differential of x all along when they decided to end the integral symbol with the term dx! Could someone please give me insight into this. Jeeze I hope I am making some kinda sense here.:-p
 
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you mean something like this?

[tex] \[<br /> \begin{array}{l}<br /> y = 3x^2 \\ <br /> \frac{{dy}}{{dx}} = 6x \\ <br /> dy = 6xdx \\ <br /> \int {dy} = \int {6xdx} \\ <br /> y = 3x^2 \\ <br /> \end{array}<br /> \][/tex]
 
yeah but i don't need to know how they work but why you can treat the dx in an integral as a differential. I know it sounds weird bear with me.
 
G01 said:
yeah but i don't need to know how they work but why you can treat the dx in an integral as a differential. I know it sounds weird bear with me.
Well generally, when you learn basic calculus such as that is stated above, without the integral sign dx on its own it's just lazy notation and not rigorously defined. It's just a bit of time and hides a bunch of results in it such as the Fundamental Theorems of Calculus:

http://mathworld.wolfram.com/FundamentalTheoremsofCalculus.html

However, dx can actually be rigorously defined on its own and is frequently used in such subject matter as Vector Calculus.
 

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