So after three semesters of being told that dy/dx IS NOT A FRACTION and that we're not allowed to separate the dy and dx, you're going to tell me "haha just multiply the dx over and integrate, it's easy! XD"?????
and ye i dont get differentials i just deal with them
Bentley Young
It's not a fraction. You can sometimes pretend, this is one of the few examples. Mathematically it's kind of dubious, but it's a nice way to remember the method. Look at it like that, there's is no deeper meaning to it.
Anthony Foster
so its on one side and then its on the other and it just works
r u sure ur just not a brainlet like me
why are we allowed to do this? what even are differentials? what is this shit
Zachary Murphy
You learned about integration by substitution? The just supose y is a composition and apply it. Your prof is probably lazy and is scared someone will ask him to explain the theorm and he will choke.
Christian Hughes
You are correct in saying that you can't just multiply by dx. But this is an easy way for brainlets to solve DE so that's why you're prof teaches it. An equivalent but better way would be to divide by y^2 and write RHS as a derivative of 1/y and then integrate.
Nicholas Scott
>Can't multiply by dx Literally why not? dx is just a small change in x nothing else
dy/dx is a limit... it's a single entity. It's misleading notation.
Lincoln Powell
cos dy/dx is a limit technically if im not mistaken so separating that "fraction" actually makes no sense
Matthew Evans
Yeah sorry I was thinking of ∆x
Angel Murphy
Leibniz was a mistake
Jonathan Wright
There exists equally rigorous constructions of the derivative where dy represents infinitesimal change and /dx represents division by an infinitesimal.
Parker Phillips
not if you want to extend to multivariable analysis, particularly if you try and construct a reasonable change of variables that reproduces the jacobian result. Unless of course you just change gears and abuse the language of differential forms out of nowhere.
Landon Robinson
hey you sound like u know what ur talkin about
help a brainlet out differential dy/dx is just a limit hunh how do u do a separable differential equation if u treat differentials like that
Levi Bennett
I would guess it's just a notation of sorts. dx just means with respect to x
Isaac Thompson
>how do u do a separable differential equation if u treat differentials like that Did you not read ?
Ultimately, multiplying by dx is just a heuristic... a "shortcut" for doing it the "real way." It has no actual valid mathematical meaning (that is, if we treat dy/dx as a limit).
Nathaniel Morales
yes but how would i do it the real way? i don't see how the stackexchange answer is suitable as he seems to play with and separate differentials as well
Austin Fisher
isn't the first derivative of (xy)^2 equal to 2x^2y + 2xy^2 ?
Camden Jones
They didn't differentiate, they just rewrote it in an equivalent form. (xy)^2 = (x^2)(y^2)
Samuel Miller
ye i'm just a mere brainlet thanks for pointing it out.
Jacob Harris
Once you proof rigorously that your symbol manipulation is valid, you can do it. Same with integration by substitution and differentiation of compositions. It's easier for people to remember that way
Nathaniel Jones
It's just notation, but you should be comfortable proving it yourself.
Wyatt Foster
...
Benjamin Flores
>implying you can't separate one limit into two [math]\lim_{x -> x'}{\dfrac{y(x)-y(x')}{x-x'}}=\dfrac{\lim_{x -> x'}{y(x)-y(x')}}{\lim_{x -> x'}{x-x'}}[/math]
James Long
that's wrong
Thomas Wilson
it's a fraction! [eqn]\frac{dy}{dx} = (xy)^2 [/eqn] now cancel the "d"s [eqn]\frac{y}{x} = (xy)^2 [/eqn] or [eqn]y = x^{-3} [/eqn]
Adam Kelly
tbf i have yet to see a case where this wouldn't work
Jace Jenkins
the fuck is wrong with you faggots, looks at OP's picture. [math]x^2 dx \neq x [/math] Simple typo but you guys should have noticed. Also as an engineer, I have yet to encounter a differential equation I can not solve by selecting a finite difference approximation and rewriting as a recurrence relation. The ability to go back and forth between dx notation and some finite difference is enough to make mincemeat of anything encountered in a college diff equ class.
Jose Rivera
U can manipulate integratipn by parts to prove 1=0.
Christian Wilson
dy/dx is not a fraction. BUT dy=(dy/dx)*dx
Elijah Torres
That still not a fraction. It's defined as the best linear aproximation, you still need to prove you can manipulate it.
Daniel Green
Can you read? I said dy/dx is not a fraction. It's a slope. dy=(dy/dx)*dx. Infinitesimal change in y is equal to the slope times infinitesimal change in x.
Brayden Taylor
>infinitesimal Define this concept properly ypu retard
Angel Martin
>infinitesimal small change. Is it really that hard to grasp?
Anthony Torres
Define small
Ayden Jackson
the distance between two points on a curve where the distance can be reasonably approximated as a straight line. >inb4 define reasonably approximated It depends on the application
Colton Anderson
One must learn first the concept of derivative, and understand it well.
Then one fucks around with dx and dy because it's literally useful, specially in Physics if one thinks of what this means physically (an infinitesimal rate of change of some quantity).
You can do the variable separation method with additional intermediate steps. For example, you take the first line, you integrate both sides of the equation in x. then you get the 2nd line.
This is also useful once you get to partial differential equations, since it will actually lead you places.
But when it's one variable, come on.
Hunter Gonzalez
>y'(x') = 0/0 genius
Connor Bell
No you retard, we are talking about proper mathenatical definition. I'll give you a definition brainlet. The diferential is the best linear aproximation of a function at a point. That means the error function over the distance between two points tends to 0 as you aproximate the point. By that means a limit which is well defined.
Justin Miller
He's an engineer, ignore him.
Aaron Brooks
>the best linear aproximation of a function at a point. >the distance between two points on a curve where the distance can be reasonably approximated as a straight line. Yep these two statements are completely different. And since you asked me to properly define infinitesimal, how about you properly define the error function for me?
Ryder Baker
See Taylor's theorem faggot.
Henry Hughes
You could have looked up infinitesimal, but you instead asked me to define it. I'm just asking the same of you. No need to get butthurt.
Robert Harris
Its also not propely deined as "small" you cuckgineer faggot.
Jeremiah Watson
are you saying infinitesimals aren't small?
Jason Brown
Im saying that small is not a rigorous mathematical concept. That's why there are a ton of equivalent definitipns that range from algebra to analysis
Jackson Nelson
what makes a concept mathematically rigorous?
Hunter Ortiz
>You can sometimes pretend, this is one of the few examples. just say you're doing a subtle change of basis already
John Edwards
It is an infinitesimal fraction, some of the time. Thinking of it as a fraction is the reason seperation of variables works with linear ODEs and why arc length works in n => 2
Aaron Turner
That's actually a rather complicated question. It's so hard to pinpoint that ypu have to bring historical concepts to some proofs. But generally, something is mathematically rigorous id you could derive it from first principles using a particual set of symbols called a "language". In most cases this principles are ZFC set theory using first order logic I think. That's why its important to write small with the delta-epsilon definition and why inequalities are crucial.
Justin Roberts
holy shit faggot just fuck off and kill yourself
Henry Turner
Thanks. I still haven't gotten a rigorous definition for the error function other than a vague reference to Taylor's theorem. Stay mad
Michael Howard
You should infer it from the name... It's a function that by definition gives you how much your linear aproximation deviates from the value of ypur function. And no, it's not given explicitly, thate important part is that it tends to 0 "rapidly" as you get close to the point. Literally just read the wikipedia entry on Taylor's theorem brainlet
Lincoln Reyes
>You should infer it from the name And yet you can't infer what small means by the name? This is my point. >it's not given explicitly Then how can it be apart of a rigorous definition? >important part is that it tends to 0 "rapidly" as you get close to the point. What do you mean by "rapidly"? It depends on the application, as I said in You're just mad that I turned out to be right from the very beginning. Try thinking critically before you criticize others.
Thomas Watson
It's just combining intuition (when it's sensible, you can think of the derivative as approximately the change in a function divided by its change in variable) and the fact that it works as claimed due to the fact that [math] \int g(f(x)) \frac{df}{dx} dx = \int g(f) df \,\,\,. [/math] Extend to a class of first order separable differential equations as: [math] y'(x) = f(y(x))*g(x) \implies \frac{y'(x)}{f(y(x))} = g(x) \implies \int \frac{y'(x)}{f(y(x))} dx = \int g(x) dx \,\,\,. [/math] Then just apply the usual result and you get that the 'dy/dx' separate into an integration over y (symbolic 'dy') and an integration over x (symbolic 'dx').
James Reyes
>And yet you can't infer what small means by the name? You fucking brainlet, small is a relative concept. For every "small" distance, you could multiply it by 0.00000000000000000000000000000000000001 to make it even smaller. And you can repeat this recursively. So at what point do you stop and say that a distance is "small" enough? Whatever you pick will be completely arbitrary, anyway, since there are an infinite number of real numbers between any two real numbers. Saying "it's just a small change dude lol" is not mathematically useful.
Brayden Martin
>small is a relative concept. >important part is that it tends to 0 "rapidly" as you get close to the point. "Rapidly" is a relative concept too. And now that I think about it, so is "close". At least my definition only relies on one relative concept.
David Bell
Nigger, I'm not going to write all the mathenatical notation. Literally just check the entry for Taylor's theorem ffs.
Daniel Barnes
How
Bentley Long
Alright, this is my first time on this board, but are you people seriously talking about advanced class in highschool/early-college level calculus this seriously?
I think this may also be my last time on this board, Jesus Christ, bunch of autists and 18 year olds.
Leo Thompson
It's not a fraction, all you are doing is integrating with respect to X and using the chain rule.
Matthew Barnes
Went to wikipedia and ctrl+f "error function" and got nothing. Also noticed that "The exact content of "Taylor's theorem" is not universally agreed upon." Why would you demand a rigorous definition from me when you are too lazy to be rigorous yourself?
Juan Harris
>You can sometimes pretend
And Veeky Forumstards continue to pretend science is as rigid as they claim it to be.
Leo Gray
F-f-fuck off, Veeky Forums!
Benjamin Stewart
We did that in leaving cert applied maths, bro. Yeah, it makes no sense for the first few years, but you work out the logic of it.
Jaxon Hill
[math]dy[/math] and [math]dy[/math] are differentials. What they are is a representation of the arbitrary slope of x at that point.
However, this by itself is not defined. [math]x[/math] is just a real number, so [math]dx[/math] has no meaning in and of itself. However, [math]\frac{dy}{dx}[/math] does have meaning, because we have a relationship between [math]x[/math] and [math]y[/math] through the function [math]f(x):=y[/math]. [math]\frac{dy}{dx}[/math] tells you how fast [math]x[/math] is changing with regard to [math]y[/math] at any point [math]x[/math] where [math]\frac{dy}{dx}[/math] is defined. If you understand
At [math]x[/math], [math]\frac{dy}{dx}[/math] is defined as [math]\lim_{t\to x}\frac{f(t)-f(x)}{t-x}[/math] when this limit is defined. Which, if you think, means we're taking smaller and smaller chunks of [math]f(x)=y[/math] compared to smaller chunks of [math]x[/math]. So if you want to think in terms of infinitesimals, (which only should be used as a heuristic), we have [math]\frac{dy}{dx}\approx\frac{\Delta y}{\Delta x}[/math].
In context, it definitely can be inappropriate to move a differential. In calculus, with a rigidly defined [math]f(x):=y[/math], [math]\frac{dy}{dx}[/math] has a definite meaning, particularly it is a function [math]f'(x)=\frac{dy}{dx}[/math] and takes explicit number values at every [math]x[/math]. To "split up" [math]\frac{dy}{dx}[/math] would take a number and try to replace it with differentials. Algebra simply doesn't work that way, it's a nonsense gesture.
However, in the case of a differential equation, we're attempting to find a function which works as a solution to an equation. As such [math]\frac{dy}{dx}[/math] has not taken on an explicit meaning. Therefore it's completely appropriate to manipulate them algebraically.
Leo Jenkins
If you wanted to understand what was told to you, you would have realized pretty quickly that the "reminder" is what he referred as an error function.
Xavier Gray
autism
Justin Perry
>after three semesters
>americuck education
Daniel Ward
I'm an engineer, faggot, what did you expect?
Christopher White
It's not a fraction in the sense you can't just go and multiply it with other shit you've got around and solve for d or whatever. It is a fraction in the sense that it's an infinitely small change in y over an infinitely small change in x. Just use fucking common sense when doing math, jesus.
Brayden Green
>3 semesters >dy/dx ain't a fraction Smh
Cooper Stewart
Faulty. Life is messy.
Jacob Nelson
>can't understand such a simple concept >can't google for an explanation >fucking christ engineers are retarded
Jason Rogers
...
Ian Clark
>IS NOT A FRACTION It isnt, it is just notation that alows you to do this things.
If you understood what dy/dx meant you would understand why you can do this.
Christian Ramirez
I've never understood differential equations beyond 1st order ODEs
It's beginning to catch up with me now since I have a module at university which is mainly based on differential equations. Hopefully by the end of this year I will understand them
Hudson Sanchez
You can't divide by differential forms.
Julian Morris
Listen, OP. Some of the explanations so far are good -- others are cancer.
No, it's not really a fraction, mathematically speaking. Intuitively speaking, dx or dy, represents some infinitesimally small difference in y or x, and can be thought of as such.
If you want a rigorous, mathematical explanation for why you can "multiply" by dx, recall the chain rule.
Landon Reyes
>he doesn't know about infinitesimals
how cute, it's a brainlet
Adam Wright
differentials are not infinitesimals.
Caleb King
"an indefinitely small quantity; a value approaching zero."
What are you on about. I'm using the term in the classical sense
Jonathan Hall
there is no such thing as an indefinitely small quantity. differentials are defined in terms of limits. they are different ideas.
Jose Anderson
Please define the size of a point in space. I'll give you a hint: it is infinitesimally small.
Landon Ward
a point has zero measure.
Parker Reyes
But it exists in space, does it not?
Ian Barnes
the length of a point is zero.
Ayden Cooper
Measure is a well defined concept
Brody Davis
Who told you it wasn't a fraction?
William Gutierrez
So please explain what is wrong with my use of the word "infinitesimal"
Ryder Gray
there is no such thing as an infinitely small numbers. [math]\frac{dy}{dx}\approx\frac{\Delta y}{\Delta x}[/math] is true but it's never equal for any [math]\Delta[/math]. You must take the limit, which is expressly different than the infinitesimal.
Cameron Ross
It may seem like a "trivial" detail to you, but infinitesimals are wrong enough to cause serious problems in mathematics, they break and make things contradictory. I know that might not matter to someone like you, but an engineer's perspective on math does not matter.
Sebastian Ward
How do you faggots not know this and end up treating derivatives as fractions?
'Litchrully' fucking kill yourselves you homosexual penis-loving engineer cucks.
Chase Garcia
underrated
Anthony King
limits go through divisions retards. Lim a/b = Lim a / Lim b
Colton Gonzalez
>Lim a/b = Lim a / Lim b >Lim x-->0 (2x)/(2x) = Lim x--> 0 2x / Lim x --> 0 2x >Lim x-->0 (2x)/(2x) = 0/0 Huh.
Aaron Myers
AFTER 3 SEMESTERS OF BEING TOLD AT PRIMARY SCHOOL THAT I CANT TAKE AWAY LARGER NUMBERS FROM SMALLER NUMBERS IM NOW GETTING TOLD THAT IM ABLE TO DO THIS?