Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Proof f'(x)/f(x)=|f(x)|

  1. May 20, 2003 #1


    User Avatar
    Gold Member

    Last edited by a moderator: Apr 20, 2017
  2. jcsd
  3. May 20, 2003 #2
    I may be being naive here, but I can't see how that's true...
    Consider f(x)=x. Then f'(x)=1

    f'(x)/f(x)=1/x which is not equal to |x|.

    Am I right in thinking this?
  4. May 20, 2003 #3
    It should probably say [inte](f'(x)/f(x))dx = ln|f(x)|.

    Try integrating cos(x) / sin(x) for example.
  5. May 20, 2003 #4
    i never saw this equation before
    if f'(x) is a derivative of f(x)...
    this equation should be wrong
    if not, then what is it??(i mean maybe have other meaning??!!)
  6. May 20, 2003 #5
    [inte](f'(x)/f(x))dx = ln|f(x)|.
    yes....i think this is the right answer
  7. May 20, 2003 #6
    I wouldn't use that site much more...

    ∫1/x2dx= tanh-1x+c?!
  8. May 20, 2003 #7


    User Avatar

    Seems to be a straightforward case of integration by substitution, doesn't it? Which is derived from chain rule...
  9. May 20, 2003 #8

    Tom Mattson

    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    It's also wrong, which is Lonewolf's point. It should be:

    ∫1/(x2+1)dx= tanh-1x+c

    Funny thing is, they got the derivative of tanh-1(x) right!
  10. May 20, 2003 #9
    Shouldn't that be

    ∫1/(1-x2)dx= arctanh x + c?
  11. May 20, 2003 #10

    Tom Mattson

    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    Oops--you're right. I got tanh-1(x) confused with tan-1(x).
  12. May 21, 2003 #11
    just differentiate ln|f(x)|.
  13. May 21, 2003 #12
    This is actually f’ (x)/f (x)=|f’ (x)/f (x)|. Let’s make some examples:

    If f (x)=x then f’ (x)=1, therefore, f’ (x)/f (x)=1/x and |f’ (x)/f (x)|=Square-root (1/x)^2=1/x

    If f (x)=x^2 then f’ (x)=2x, therefore, f’ (x)/f (x)=2/x and |f’ (x)/f (x)|= Square-root (2/x)^2=2/x

    How about if f (x)= -x then f’ (x) = -1, therefore, f’ (x)/f (x)= -1/-x=1/x and again |f’ (x)/f (x)| =1/x
    Obviously f’ (x) / f (x) is always equal to |f’ (x)/f (x)|

    In general: if f (x) = cx^n then f’ (x) = ncx^(n-1) so f’ (x)/f (x) = ncx^n/cx^(n+1)= nx^n/x^(n+1) =nx^n/(x^n)*x=n/x

    So f’ (x)/f (x) for x^3 is 3/x or x^4 is 4/x and so on (always n/x). This equation states that there is always a singularity at x and it exists at first and third quadrants. I hope this was helpful:wink:
    Last edited by a moderator: May 21, 2003
  14. May 21, 2003 #13
    What about f(x) = 1/x?

    f'(x) = -1/x2

    f'(x)/f(x) = -1/x

    This is not equal to |f'(x)/f(x)|
  15. May 21, 2003 #14
    For the expression:


    What does the "∫" mean. Tell me all about it, please.
  16. May 21, 2003 #15
    It's an integral sign. Integration is a part of calculus where we find areas of regions, naively speaking. There's no real general method of performing integration, but there are certain standard integrals. The site listed above gives some examples of standard integrals, although a few of them are wrong. I'm sure someone can give a more precise definition than I have.
  17. May 21, 2003 #16
    another mistake

    [inte] tan hx
    =/= ln|cos hx| + C
    = ln|sec hx| +C
    (another mistake from the website)

    For example, a function f(x) = x^2
    The first derivative of f(x), f'(x) = 2x

    Now we are given f'(x) = 2x , how can we find f(x)? We use integration.
    [inte]2xdx = x^2 + C (where C is the constant of integration)

    Defination: Let f(x) be defined on [a,b]
    If F'(x)=f(x), then F is called the primitive function of f and we write it as F(x)=[inte]f(x)dx.
    Since F(x)+C is also a primitive function of f, therefore [inte]f(x)dx= F(x) + C where C is the constant of integration
  18. May 21, 2003 #17
    Not all functions have a closed form integral. One such famous one is the probability density function of the Gaussian distribution

  19. May 21, 2003 #18
    1/x or better said x-1 is not in the form of cxn, but it is in the form of cx(-n), which generally exists in the second and forth quadrants with equation of –n/x. In this case we would have -|f’ (x) / f (x)|. :wink:
  20. May 21, 2003 #19
    You didn't say what n had to be. f(x) represents a general function. Your 'proof' doesn't cover a general function, only polynomials.
    Try f(x) = sin(x) for size :wink:
  21. May 21, 2003 #20
    Please read my original post carefully, I said, “In general: if f (x) = cx^n then…”
    I didn’t have to, n can be anything, and however, n is not -n. I did not say if f (x) = cx -n
    Exactly!! I think you got it now.

Share this great discussion with others via Reddit, Google+, Twitter, or Facebook