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Homework Help: Trying to understand rewritten equation

  1. May 2, 2012 #1

    I am hoping someone can help me understand something. I was watching one of the MIT OCW videos and it ran through the following

    Δf/Δx = (1/(x0x - 1/x0)) / Δ0

    can be rewritten as

    Δf/Δx = 1/Δx (x0-(x0x) / (x0x)x0)

    But I am not sure how this was done.

    I hope someone can help me with this.

    Thanks in advance.

  2. jcsd
  3. May 2, 2012 #2


    Staff: Mentor

    What you wrote above is very confusing. I believe that what you saw looked more like this:

    Δf/Δx = (1/(x0+Δx) - 1/x0)) / Δx

    Using LaTeX makes it even clearer.
    $$Δf/Δx = \frac{\frac{1}{x_0 + Δx} - \frac{1}{x_0}}{Δx}$$

    What is apparently happening is that they are finding the derivative of f(x) = 1/x, at the point x0.

    The key to simplifying the above is being able to combine 1/(x0 + Δx) and 1/x0. To do this, find the common denominator and combine the two terms using that denominator.

  4. May 2, 2012 #3

    And thanks for the reply.

    Yea, sorry about writing it like I did ..I am not too familiar with LaTeX.

    Well, the common denominator is x0, which will give the x0-(x0x)
  5. May 2, 2012 #4


    Staff: Mentor

    No, the common denominator is NOT x0.

    If you had to add 3/5 and 1/(5 + 3) would you say that the common denominator was 5?
  6. May 2, 2012 #5


    Staff: Mentor

    Mod note: Moved from Precalc section.
    Last edited: May 2, 2012
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