Leibniz notation giving me a headache

  • Context: Undergrad 
  • Thread starter Thread starter Rib5
  • Start date Start date
  • Tags Tags
    Leibniz Notation
Click For Summary

Discussion Overview

The discussion revolves around the interpretation and application of Leibniz notation in calculus, particularly in the context of derivatives and integrals. Participants explore the meaning of expressions like \(\frac{dy/dx}{y-900}\) and \(\frac{d}{dx} \ln|y-900|\), addressing confusion over the notation and its implications in solving differential equations.

Discussion Character

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

Main Points Raised

  • One participant questions how \(\frac{dy/dx}{y-900}\) can equal \(\frac{d}{dx} \ln|y-900|\), noting confusion over the notation.
  • Another participant suggests that the solution to the ODE might be to multiply both sides by \((x-900)\) and integrate, raising the possibility of a typo or misunderstanding in the problem statement.
  • A different participant explains that integrating a differential leaves the original function, but expresses uncertainty about the transition between different notations.
  • One participant introduces the chain rule, stating that it implies \(d/dx (\ln(f)) = (1/f) df/dx\), but others express confusion about the distinction between \(dy/dx\) and \(d/dx\).
  • Another participant reflects on the variability of notation in calculus, mentioning experiences with different styles and the lack of consensus on the meanings of \(dx\) and \(dy\).
  • A participant discusses the concept of analysis in calculus, mentioning non-standard analysis and the treatment of \(dx\) and \(dy\) as variables in certain contexts.
  • One participant clarifies that \(dy/dx\) is a variable while \(d/dx\) is an operator, drawing an analogy to programming concepts.
  • Another participant emphasizes that \(y\) is an arbitrary symbol representing a function, and discusses the relationship between \(dy/dx\) and \(d/dx\) in terms of derivatives.

Areas of Agreement / Disagreement

Participants express varying levels of understanding and confusion regarding Leibniz notation, with no clear consensus on its interpretation or application. Some agree on the definitions of \(dy/dx\) and \(d/dx\), while others continue to seek clarity on their differences.

Contextual Notes

Participants highlight the complexity and variability of calculus notation, indicating that different contexts may lead to different interpretations. There is mention of potential typos or misunderstandings in problem statements, as well as the informal use of notation in physics.

Rib5
Messages
59
Reaction score
0
----EDIT------
In my original post I totally messed up my variables. Here I'll get straight to the point.

I guess what I am really asking is how is [tex]\frac{dy/dx}{y-900}[/tex] equal to [tex]\frac{d}{dx}[/tex]ln|y-900|

I know that the integral of [tex]\frac{1}{y-900}[/tex] is ln|y-900| but I don't understand how the notation works where one side has dy/dx and the other side just has d/dx
 
Last edited:
Physics news on Phys.org
Yes what they are doing seems very confusing. In particular, where did t come from? It would seem obvious that the solution to your ode would be just to multiply both sides by (x-900) and then integrate to give y as a quadratic function of x. Is it possible that there is a typo, or a mistake in how you read the question?
 
I don't know about the first part but for the second part just remember that Integrating a differential just leaves what's being being differentiated:

[tex]y = x^2[/tex]
[tex]\int \frac{dy}{dx} dx = y = x^2[/tex]

If the Leibniz notation is giving you problems I think you can just change it to the hm..American Style ? my teacher calls it that...

so you'd get:

[tex]\frac{y'}{x-900} = \frac{1}{2}[/tex]

[tex]y=ln|x-900|[/tex]

[tex]y'=(ln|x-900|)'=\frac{1}{x-900}[/tex]

which doesn't seem to be the same since the top y' on the first formula seems to disappear going to the second one... But then again I have no knowledge of Diffy Eq's.

Also did they integrate w.r.t. x on the LHS but w.r.t. t on the RHS? on the final step of y'=1/2
 
do you know the chain rule?

it impies that d/dx (ln(f)) = (1/f)df/dx.
 
Last edited:
I know the chain rule and I know how to do the computations, what I do not understand is what exactly the dy/dx and then later d/dx means.

My problem was when I went through my calculus classes I always used f'(x) or y' for my notation. And then with integrals f__dx I just learned how to use it not what it actually means. It seems like there are so many different meanings for leibniz notation and no one really agrees on it. Today I asked a physics teacher how to solve a problem and he used dx and dy almost like variables and put one to one side and one to the other than slapped on an integral sign and that worked too...
 
That's because you can do analysis/calculus in a variety of ways -- to be really fancy, "analysis is not fully identified in first order logics". Once you get to learn "real" analysis, you'll come across a metric buttload of things like epsilon and deltas and taking limits as things tend to zero, or this or that. There's another approach, ironically called "non-standard analysis", in which we extend the real numbers, and have infinitesimals that behave rather like what Leibniz notation suggests, except with actually self-consistent algebraic rules.

Personally, I'm a physicist, so I just throw dx and dy about like they're variables, occasionally throwing in an integral sign to the mix to make things correct. It's all really just shorthand for the long, formal manipulations. Dodgy shorthand, to be sure.
 
genneth said:
That's because you can do analysis/calculus in a variety of ways -- to be really fancy, "analysis is not fully identified in first order logics". Once you get to learn "real" analysis, you'll come across a metric buttload of things like epsilon and deltas and taking limits as things tend to zero, or this or that. There's another approach, ironically called "non-standard analysis", in which we extend the real numbers, and have infinitesimals that behave rather like what Leibniz notation suggests, except with actually self-consistent algebraic rules.

Personally, I'm a physicist, so I just throw dx and dy about like they're variables, occasionally throwing in an integral sign to the mix to make things correct. It's all really just shorthand for the long, formal manipulations. Dodgy shorthand, to be sure.

This sort of helps ease my mind since I am always wondering, is this ok? Can we treat this as a variable sometimes and sometimes not. It doesn't seem like there is some real rule to follow... unless I guess I went further into math and did that other stuff.

So for now I guess I just have to learn when it is ok and when it is not and just accept it? I'm a chemistry major just minoring in math so I don't really need any more math than just the application portion of it, at least for now.
But I still don't understand the difference between dy/dx and just d/dx
 
Strictly speaking, dy/dx is a variable (taken as a whole, no splitting!) and d/dx is an operator. If you know anything about programming, operators are like higher-order functions -- they take functions and spit out another one.
 
Y s just some differentiable function. It’s an arbitrary symbol used to represent a function. As long as you get that in your head you will be fine.

dy/dx by definition is the derivative of y with respect to x.

But what if y = x^2 + e^x

Then couldn’t we write dy/dx = 2x + e^x or d(x^2 + e^x) = 2x + e^x?

Putting d/dx is just short hand for the second way we write it to mean take the derivative of what ever function follows.
 
  • #10
Thanks for all the help guys, I'm pretty sure I get it now :)
 

Similar threads

  • · Replies 6 ·
Replies
6
Views
3K
  • · Replies 1 ·
Replies
1
Views
3K
  • · Replies 12 ·
Replies
12
Views
3K
  • · Replies 10 ·
Replies
10
Views
5K
  • · Replies 8 ·
Replies
8
Views
3K
  • · Replies 22 ·
Replies
22
Views
3K
  • · Replies 13 ·
Replies
13
Views
3K
  • · Replies 4 ·
Replies
4
Views
2K
  • · Replies 4 ·
Replies
4
Views
2K
  • · Replies 3 ·
Replies
3
Views
2K