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: Help me finding inverse function

  1. Oct 3, 2007 #1
    What is the inverse of this function [tex]f(x) = x^3 + 2x[/tex]?

    ***************************************

    [tex]f(x) = x^3 + 2x[/tex]

    [tex]y = x^3 + 2x[/tex]

    interchange [tex]x \Leftrightarrow y[/tex]

    [tex]x = y^3 + 2y[/tex]

    and it's a dead end to me ...
     
  2. jcsd
  3. Oct 3, 2007 #2

    Integral

    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    That will not get you where you need to be. Rather then simply interchanging x and y, you need to chang your expression for f(x) as F(y). A simple example:

    f(x)= 3x + 5

    y = 3x + 5

    y - 5 = 3x

    y/3 - 5/3 = x

    x = y/3 - 5/3

    so now you have x = F(y)
     
  4. Oct 3, 2007 #3

    HallsofIvy

    User Avatar
    Science Advisor

    What you need to do is solve the equation x3+ 2x= y for x. Thats solving a fairly general cubic equation. Do you have any reason to think that the inverse can be written in a simple form?
     
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook