Dy/dx as Fraction: A-Level Maths/Further Maths Explanation

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Discussion Overview

The discussion centers on the interpretation of derivatives, specifically the notation dy/dx, in the context of A-Level Maths and Further Maths. Participants explore whether derivatives can be treated as fractions and the implications of this perspective, particularly in relation to the Chain Rule.

Discussion Character

  • Debate/contested
  • Conceptual clarification
  • Mathematical reasoning

Main Points Raised

  • One participant expresses confusion about being instructed not to view derivatives as fractions while learning the Chain Rule, where dy/dx appears to behave like a fraction.
  • Another participant argues that since dx and dy are not numbers, dy/dx cannot be regarded as a fraction, emphasizing the distinction between fractions and derivatives.
  • A different viewpoint suggests that since dx and dy can be considered infinitesimals, dy/dx can indeed be treated as a fraction, and this aligns with the behavior observed in the Chain Rule.
  • One participant introduces the concept of differentials as sections of a line bundle, arguing that under certain conditions, the quotient of two vectors in a one-dimensional vector space can yield a numerical value, thus supporting the notion of treating derivatives as fractions.
  • Another participant reiterates the idea of differentials and mentions the hyperreals, suggesting a specific mathematical framework that allows for the interpretation of dy/dx as a fraction.

Areas of Agreement / Disagreement

Participants express differing views on whether derivatives can be treated as fractions. There is no consensus, as some maintain that derivatives should not be viewed as fractions, while others argue that they can be under certain mathematical interpretations.

Contextual Notes

The discussion highlights the complexity of interpreting derivatives and the varying mathematical frameworks that participants reference, including infinitesimals and vector spaces. There are unresolved assumptions regarding the definitions and implications of treating derivatives as fractions.

barnaby
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I started calculus in September (as part of A-Level Maths/Further Maths), and we've been told time after time not to look at derivatives as fractions.

We recently did the Chain Rule, and we were told that a way to remember it was that if dy/dx = dy/du * du/dx, then the 'du's 'cancel out' - which flies in the face of not looking at derivatives as fractions. My teacher then told us that we would eventually see how derivatives could be treated like fractions...

How and why can we do this?
 
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barnaby said:
I started calculus in September (as part of A-Level Maths/Further Maths), and we've been told time after time not to look at derivatives as fractions.
Remember that a "fraction" is a relation between two NUMBERS.
Since you cannot regard "dx" and "dy" as numbers, it follows that the symbol dx/dy cannot be regarded as a fraction.
 
arildno said:
Remember that a "fraction" is a relation between two NUMBERS.
Since you cannot regard "dx" and "dy" as numbers, it follows that the symbol dx/dy cannot be regarded as a fraction.
Actually, dx and dy are infinitesimal numbers, so it follows that the symbol dy/dx can be regarded as a fraction. Furthermore, it is not a coincidence that the chain rule works as though something is being canceled...because something is being canceled.

See, for example, chapter 2 of
http://www.math.wisc.edu/~keisler/foundations.pdf
 
actually dy and dx are differentials, i.e. sections of a certain line bundle, hence functions whose values are elements of a one dimensional vector space. now as long as a vector w is non zero, the quotient v/w of two vectors in a one dimensional vector space IS a number. so fractions do make sense and have numbers as values, as long as the top and bottom of the fraction are vectors in the same "line".

there is actually a good, elementary explanation of differentials in the beginning of the classic diff eq book by tanenbaum and pollard.
 
mathwonk said:
actually dy and dx are differentials, i.e. sections of a certain line bundle, hence functions whose values are elements of a one dimensional vector space..

...that one dimensional space being the hyperreals, to be more specific, which is what I said.
 

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