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Math Help - Contradictory Derivative

  1. #1
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    Contradictory Derivative

    Let's say f(x) = x^2.
    This would give f'(x) = 2x.
    Now we change the definition of f(x) = x^2 to f(x) = x + x + x... up to x times.
    If we now differentiate it with respect to x, we will get f'(x) = d(x)/dx + d(x)/dx + d(x)/dx ....up to x times. And this sums up to x. Thus according to new definition of f(x), the derivative f'(x) would be x and not 2x.
    Though we all know that there is some gap in calculating the derivative in second method, what is the missing thing here? How can it be captured to cover every aspect?
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  2. #2
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    Re: Contradictory Derivative

    Hey vamosromil.

    The subtle difference is that 1) this only applies if x is an integer and 2) such a summation assumes that we sum up a known amount of times (i.e. n*x where n is fixed).

    If you add up d(x)/dx n times you get n which is what we expect.

    The derivative of nx is n and does not depend on x at all.

    You can't do what you did because what you are doing (and the way you are doing it) is ill-defined. If you are summing up a fixed amount of times is just (x + x + ... + x) n times and not "x" times (what if x is 1.123123798123978123 or pi)?

    You have to be careful with how you specify things.
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    MHF Contributor FernandoRevilla's Avatar
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    Re: Contradictory Derivative

    Quote Originally Posted by vamosromil View Post
    Now we change the definition of f(x) = x^2 to f(x) = x + x + x... up to x times.
    So, f(\sqrt{2})=\sqrt{2}+\sqrt{2}+\ldots +\sqrt{2} up to \sqrt{2} times?


    Edited: Sorry, I didn't see chiro's post.
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    Re: Contradictory Derivative

    I wonder how can you write down 2.453+2.435+?... exactly 2.435 times?
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    Re: Contradictory Derivative

    Hello everyone, I see there are some typical questions being asked. I posted this problem on another forum too. And the first question of writing a real number other than integer that many number of times has also been addressed. But the core explanation is yet to be found..here is the link for that forum: Contradictory Derivative
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  6. #6
    Forum Admin topsquark's Avatar
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    Re: Contradictory Derivative

    Quote Originally Posted by vamosromil View Post
    But the core explanation is yet to be found..here is the link for that forum: Contradictory Derivative
    What "core explanation?" Asked and answered.

    -Dan
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