1. Limited time only! Sign up for a free 30min personal tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Exponential functions

  1. Apr 9, 2007 #1
    1. The problem statement, all variables and given/known data
    Solve the equation:
    (1/9)^m = 81^m + 4

    2. Relevant equations
    I know how to do a similar equation:
    9^2p = 27^p-1

    3^4p = 3^3p-1

    cancel out the 3's because the base is the same.

    4p = 3p-1
    -3p -3p

    1p = -1

    3. The attempt at a solution
    I know how to do ones like that but when it comes to this I start to struggle with fractions. :'(.

    I'm totally lost and cant even make an attempt to this equation. I'm not asking for the answer but just a maybe the first step or something to get me on the right path.
  2. jcsd
  3. Apr 9, 2007 #2
    How about converting some terms to powers of 9?
  4. Apr 9, 2007 #3
    [tex] (\frac{1}{9})^m = 81^{m+4} [/tex]

    [tex] (9^{-1})^m = (9^2)^{m+4} [/tex]

    [tex] (9)^{-m} = (9)^{2m+8} [/tex]

    [tex] -m = 2m+8 [/tex]

    [tex] -8 = 3m [/tex]

    [tex] m = - \frac{8}{3} [/tex]
  5. Apr 9, 2007 #4


    User Avatar
    Homework Helper
    Education Advisor
    Gold Member


    The way you manage your notation is very important: you wrote
    which showed your original equation of difficulty to be very different from what you meant. In simple text-based expressiveness, you needed proper grouping saying (1/9)^m = 81^(m+4); The way you first wrote it, the "4" was not included as part of the exponent.
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Similar Discussions: Exponential functions
  1. Exponential Function? (Replies: 4)

  2. Exponential function (Replies: 1)

  3. Exponential function (Replies: 3)

  4. Exponential Functions (Replies: 2)

  5. Exponential Functions (Replies: 4)