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

E^x+x = 5

  1. Nov 17, 2011 #1
    1. The problem statement, all variables and given/known data


    2. Relevant equations

    Lambert W-function?

    3. The attempt at a solution

    I moved everything around and got: Ln(5-x)=5....
    It doesn't really help.
    I looked at wolframalpha, and it said I need the Lambert W-function (no clue what that is).

    Can this equation be solved Without graphing it, only using algebraic methods?
  2. jcsd
  3. Nov 17, 2011 #2


    User Avatar
    Science Advisor
    Homework Helper

    Re: e^x+x=5

    Nope, can't be solved with algebraic methods. It's Lambert W or a numerical solution.
  4. Nov 17, 2011 #3


    User Avatar
    Staff Emeritus
    Science Advisor
    Homework Helper

    Re: e^x+x=5

    You might want to check your rearrangement of the original equation.

    What is ln(e^x)?
  5. Nov 18, 2011 #4
    Re: e^x+x=5

    What you want to do. IF you want to find exact solution then i think it is task of machines(wolfarmalpha) and too tough for me.
    If you want to know the number of solutions this equations will have then. It is possible.
    draw curve of e^x and 5-x.rough e^x curve is as a^x(a>0). and 5-x is straight line with slope -1. So these two curve will intersect each other at one point hence it will have one solution.
  6. Nov 18, 2011 #5


    User Avatar
    Homework Helper

    Re: e^x+x=5

    You can get the root of the equation with an iterative method for the desired accuracy. Rearrange the equation:


    Choose an x0 <5, determine x1=ln(5-x0) as the next approximation. Substitute again, to get the next x and repeat the procedure: xk+1=ln(5-xk)

    Starting with x=1, the following values are obtained: 1.386, 1.284, 1.312, 1.305, 1.307, 1.3064, 13066, 1.3066

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