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

Homework Help: Finding x

  1. Mar 8, 2010 #1

    Im trying to rearrange to find x from the below (all other values, A, b and c known)

    [tex] A = bx / 1-(1+x)^-c [/tex]

    Below denominator ending reads (1+x)^(-c)

    I've rearranged but to no avail, I'm unsure how to isolate x

    Any input much appreciated

  2. jcsd
  3. Mar 8, 2010 #2


    User Avatar
    Science Advisor
    Homework Helper

    Hello Toby! :smile:

    (you needed to put the index in curly brackets, {-c}, since it had more than one character … alternatively, try using the X2 tag just above the Reply box :wink:)

    This is the same as (1 + x)-c = 1 - (b/A)x …

    I don't think there is a "simple" solution.

    But why are you looking for one?
  4. Mar 8, 2010 #3

    I've never come up against an equation like this before, just curious whether it can be solved for x ? (new skills)

    Is there a general solution to this type of equation ?

    Can you point me in the right direction ?

    Thanks very much
  5. Mar 8, 2010 #4


    User Avatar
    Science Advisor
    Homework Helper

    I don't think so.

    Real life just isn't that convenient! :redface:
  6. Mar 8, 2010 #5
    Thanks anyway

    Anybody else think they know the answer ?
  7. Mar 8, 2010 #6


    User Avatar
    Homework Helper

    Really, the problem is very complicated to solve, and for most [itex]c\geq 4[/itex] it's likely impossible to solve!

    There is a proof that says for polynomials of degree 5 or higher, there is no way to solve the equation in its general case like quartics and below have been.
  8. Mar 9, 2010 #7

    I'm using Excel to approximate x for known values of A, b, c

    Thanks anyways
  9. Mar 9, 2010 #8
    Just a note

    Excel Goal Seek function solved my function to 4 decimal places, good tool
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook