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Increasing function

  1. Sep 16, 2013 #1
    Hi! I have the following question.
    Let [itex]f[/itex] be continuous on [itex][0,\infty[[/itex], [itex]f(0)=0[/itex], [itex]f^\prime[/itex] exists on [itex]]0,\infty[[/itex], and [itex]f^\prime[/itex] is increasing on [itex]]0,\infty[[/itex].

    the question is to prove that the following function is increasing that is [itex]g(x)=f(x)/x[/itex] on [itex]]0,\infty[[/itex].

    I tried to show that the first derivative is positive but I did not succeed to use the monotonicity of [itex]]f^\prime[[/itex]
  2. jcsd
  3. Sep 16, 2013 #2
    If [itex]f'[/itex] is increasing, what does that say about the shape of [itex]f[/itex]?

    Can you say anything comparing [itex]f'[/itex] to [itex]g[/itex]?
  4. Sep 18, 2013 #3
    If the derivative is increasing that mean the original function is concave up, but how can that solve the question. I did not get it.
  5. Sep 18, 2013 #4
    Good first step.

    2) Now, can you say anything about the comparison between [itex]f'[/itex] and [itex]g[/itex]? Must one of them always be bigger than the other?

    3) Is there a way to express [itex]g'[/itex] in terms of [itex]f'[/itex] and [itex]g[/itex]?
  6. Sep 18, 2013 #5
    I can't resist being... well, an economics nerd. This has a very clear meaning in public finance, for the case where [itex]f(x)[/itex] represents the number of tax dollars a person who makes income [itex]x[/itex] is required to pay. It says that if the marginal tax rate (i.e. how much you pay on your last dollar earned) is increasing, then taxation is progressive, i.e. higher income folks pay a higher overall tax percentage.
  7. Sep 18, 2013 #6


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    I would recommend drawing a picture if you're confused about how to start. Make up your favorite f(x) function that fits the requirements and draw a graph. The value of g(x) can be represented by the slope of a line that you can draw on your f(x) graph - if you can figure out what line it is the slope of you are well on your way to solving the problem
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