The sin(a+b) formula and the product rule for derivatives

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Discussion Overview

The discussion revolves around the perceived similarities between the sine addition formula and the product rule for derivatives. Participants explore the nature of these similarities, questioning the underlying principles that might connect the two mathematical expressions.

Discussion Character

  • Exploratory
  • Debate/contested
  • Mathematical reasoning

Main Points Raised

  • One participant suggests that the sine addition formula and the product rule for derivatives share a structural similarity, noting the presence of two factors in both expressions.
  • Another participant challenges the idea of similarity, asking for clarification on what is meant by "similar."
  • A participant acknowledges that the term "similar" may be too strong but points out that both formulas have a pattern where the functions involved alternate, leading to a vague resemblance.
  • There is a mention of the binomial theorem as a possible connection to the observed patterns, although this remains speculative.
  • One participant provides a specific example using sin(x) to illustrate their point, suggesting that the relationship between the two formulas might be more about identity than a fundamental principle.
  • Overall, participants express curiosity about whether other formulas might follow a similar pattern, indicating a desire for broader insights.

Areas of Agreement / Disagreement

Participants do not reach a consensus on the nature of the similarities between the sine addition formula and the product rule for derivatives. There are competing views on whether the observed patterns signify a deeper connection or are merely superficial.

Contextual Notes

Participants express uncertainty about the fundamental reasons behind the perceived similarities and the potential relevance of the binomial theorem. The discussion remains open-ended regarding the exploration of other formulas that might exhibit similar characteristics.

kotreny
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Why are they similar?

sin(a+b)=sin(a)cos(b)+cos(a)sin(b)

d/dx (f(x)*g(x))=f(x)g'(x)+g(x)f'(x)

Somewhere on this very site there was mention of this, I believe, though I can't remember where. Maybe I'm delirious.
 
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I don't see how they are very similar actually? What do you mean by "similar" ?

I think perhaps what you saw was that the formula for repeated differentiation of products was "similar" to the binomial theorem.
 
Admittedly, the word "similar" might have been too strong to describe the pattern I see. Just the vague shapes of the formulas are the same; everything else is quite different.

sin(a+b)=sin(a)cos(b)+cos(a)sin(b)

The two addends each contain two factors. The same functions are used in both factors (sin, cos), although the "subjects" alternate (a, b forgive the terminology.)

d/dx (f(x)*g(x))=f(x)g'(x)+g(x)f'(x)

In this case the functions are :nothing: and :prime:. The subjects are f(x) and g(x).

Yes, a very general pattern, but I feel it is like the similarity between a frog and a giraffe: They are both animals, with a heart, digestive system, etc. Maybe some basic property of arithmetic applied at the very beginning of the derivations for these somehow did this. Maybe it has something to do with the binomial theorem. There might be other formulas following this pattern too, in which case I'd be happy to know them. I'm looking for a broad, underlying principle.
 
kotreny said:
Why are they similar?

sin(a+b)=sin(a)cos(b)+cos(a)sin(b)

d/dx (f(x)*g(x))=f(x)g'(x)+g(x)f'(x)

Somewhere on this very site there was mention of this, I believe, though I can't remember where. Maybe I'm delirious.

Let's see for a simple case: If you call [tex]sin(x) = f(x)[/tex] and also [tex]sin(x) = g (x)[/tex]

[tex]sin(x+x)=sin(x)cos(x) + cos(x)sin(x)[/tex] is really like
[tex]\quad \quad \quad = f(x)g'(x) + g(x)f'(x)[/tex]

and this is equal to [tex]d/dx (f(x) g(x) )[/tex] as you said... But that makes sense, since

[tex]\frac{d}{dx} sin^2 (x) = 2 sin(x) cos(x) = sin(2x) = sin (x+x)[/tex]

Apart from these simple identities, I don't see a fundamental reason for this similarity.
 
Last edited:

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