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

2^x=x^5 anyway to solve this?

  1. Aug 28, 2012 #1
    anyway to solve this? if so what level of math is needed?
     
  2. jcsd
  3. Aug 28, 2012 #2

    micromass

    User Avatar
    Staff Emeritus
    Science Advisor
    Education Advisor
    2016 Award

  4. Aug 28, 2012 #3
    Re: 2^x=x^5

    what if you logged both sides using log with 2 power base?

    you get x = 5 lnx

    is that easier to solve?
     
  5. Aug 28, 2012 #4

    uart

    User Avatar
    Science Advisor

    Re: 2^x=x^5

    As micromass said, you can't really solve that type of equation algebraically. In some simple case you might just "spot" a solution by inspection (eg 2^x = x^2; x=2 or x=4).

    You can solve the equation numerically or manipulate the Lambert-W function into a solution.

    The Lambert-W function W(x) is defined as the solution (w) to [itex]we^w = x[/itex].

    BTW. How many real solutions are you expecting to get for that equation?
     
  6. Aug 28, 2012 #5

    uart

    User Avatar
    Science Advisor

    Re: 2^x=x^5

    No, there's still no algebraic solution.

    You're best to write it as [itex]e^{ax} = x^5[/itex] and then take the fifth root of each side. (a = sqrt(2) btw).

    [itex]x e^{-ax/5} = 1[/itex]

    From here you can fairly easily manipulate it into Lambert's equation.
     
  7. Aug 28, 2012 #6
    Re: 2^x=x^5

    all of them....?
     
  8. Aug 28, 2012 #7

    Mentallic

    User Avatar
    Homework Helper

    Re: 2^x=x^5

    Can you deduce how many there are?
     
  9. Aug 28, 2012 #8
    Re: 2^x=x^5

    2 if you use calculus it seems atleast according to wolfram. been looking into the lambert w function seems waay out of my league for now. seems there are no simple answer with this. but it sure looks simple.
     
  10. Aug 28, 2012 #9
    Re: 2^x=x^5

    Solve it graphically!!! A picture is worth a thousand words.
     
  11. Aug 28, 2012 #10

    Mentallic

    User Avatar
    Homework Helper

    Re: 2^x=x^5

    [itex]x=\cos(x)[/itex] might also look simple, but it too cannot be solved algebraically. They're called transcendental equations.
     
  12. Aug 28, 2012 #11

    uart

    User Avatar
    Science Advisor

    Re: 2^x=x^5

    Yes that is correct, there are two real solutions. For some fairly small positive value of "x", x^5 will exceed 2^x, but it is important to understand that for large "x" that 2^x will eventually exceed x^5 (or any power of x for that matter).

    BTW. From my previous post [itex]x e^{-ax/5} = 1[/itex], so [itex](-ax/5) e^{-ax/5} = -a/5[/itex]. Can you see that this is exactly in the form of Lambert's equation. Of course this is only of use to you if you have software or tables or whatever that solves Lambert's equation.
     
    Last edited: Aug 28, 2012
  13. Aug 28, 2012 #12

    uart

    User Avatar
    Science Advisor

    Re: 2^x=x^5

    There is a simple numerical solution you can do on just a basic scientific calculator, and it's particularly easy if it's got an "ans" button and can automatically repeat the previous calculation (many do this by just repeatedly pressing "=" for example).

    Rearrange the equation to [itex]x = (2^x)^{0.2}[/itex] and start with an initial "guess" of say x=2. You'll find that repeated iterations of this equation converge to the first solution fairly quickly. This is called "fixed point iteration" btw.

    For the second solution you can rearrange it into [itex]x = \log(x^5)/\log(2)[/itex], with some larger initial guess for "x" this should converge fairly quickly to the second solution.

    For fixed point iteration you just keep repeating the calculation, each time using the newest value of "x" in the right hand side of the equation.
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook




Similar Discussions: 2^x=x^5 anyway to solve this?
  1. Solve x^x=x (Replies: 7)

  2. Solve e^x+2x-5=0 (Replies: 11)

Loading...