We usually denote derivative like this: f'(x) where f is a function and x the variable, right.
Let me from here on use the following notation for simplicity: D(f). If f(x) = x^2 then I write directly D(x^2) for its derivative, may I do this?
Okay, everyone knows that D(x^2) = 2*x, right. There is no argue about this.
So if I 'prove' that D(x^2) = x it should obviously be some error in the 'proof'.
But look at this:
D(x^2) =
= D(x + x + x + ...) {with x number of terms} =
= D(x) + D(x) + D(x) + ... {with x number of terms} =
= 1 + 1 + 1 + ... {with x number of terms} =
= x
Hence D(x^2) = x ???
This 'proof' is obviously wrong somewhere, but where is the error to be found?
Interesting. There seems to something funny with the assumption that X^2 = X + X + X .... (for X terms). I see that this works, sort of, but what if x was a measurement that had units attached, like meters. Then (5m)^2 is not equal to 5m + 5m + 5m + ... The units are inconsistent. And how would you have 5m terms?
Originally posted by FabianFnas We usually denote derivative like this: f'(x) where f is a function and x the variable, right.
Let me from here on use the following notation for simplicity: D(f). If f(x) = x^2 then I write directly D(x^2) for its derivative, may I do this?
Okay, everyone knows that D(x^2) = 2*x, right. There is no argue about this.
So if I 'prove' that D(x^2) = x ...[text shortened]... = x ???
This 'proof' is obviously wrong somewhere, but where is the error to be found?
That's because you don't derive the x number of terms.
Originally posted by Palynka That's because you don't derive the x number of terms.
No? I did, didn't I?
D(3*x) = D(x+x+x) = D(x) + D(x) + D(x) = 1 + 1 + 1 = 3 Right?
This could be written
D(three times x) = D(three terms of x) = D(x) three times = 1 three times = 3 Right?
Originally posted by FabianFnas No? I did, didn't I?
D(3*x) = D(x+x+x) = D(x) + D(x) + D(x) = 1 + 1 + 1 = 3 Right?
This could be written
D(three times x) = D(three terms of x) = D(x) three times = 1 three times = 3 Right?
So of course we can derive x number of times.
Sorry if I wasn't clear, I meant that the x number of times is also variable (unlike 3 in your example).
Originally posted by FabianFnas I'm not sure myself where the error is to be found in my 'proof', though I understand that there must be an error somwhere.
So, this is the error, that x is a variable in itself, and not a constant? Then I thank you, this has boggled my mind for a while...
Yes, D(x.x)=D(x+x.+.. (x times)) is different than x.D(x)=D(x)+D(x)+...(x times).
If you look at it as a derivative of a product, you'll have D(x).x+x.D(x)=2x
Edit - Intuitively, you can see that a change in x, has a double effect on x+x..(x times): first, each element of the sum changes and secondly the number of elements also changes.