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Quick algebra question

  1. Nov 23, 2014 #1
    1. The problem statement, all variables and given/known data

    4*(5^x)-3(0.4^2x)=11

    In case it doesn't make sense, in the attempted solution it's written out better

    2. Relevant equations


    3. The attempt at a solution

    http://imgur.com/SnpFQZV

    Thanks!
     
    Last edited: Nov 23, 2014
  2. jcsd
  3. Nov 23, 2014 #2

    Mark44

    Staff: Mentor

    In the image, which is difficult to read, you have a different problem:
    ##4 \cdot 5^x - 3 \cdot 0.4^{2x} = 11##
    Is what you have in the image the equation you need to solve? Also, are you sure you have written the problem correctly? Both problems seem more difficult than the usual kinds of problems of this type.
     
  4. Nov 23, 2014 #3
    Yes! I'm really bad with typing out equations, but that's exactly it!
     
  5. Nov 23, 2014 #4

    Ray Vickson

    User Avatar
    Science Advisor
    Homework Helper

    It would have been 100% clear if you had put parentheses around the '2x', like this:
    4 * 5^x -3 * (0.4)^(2x).
     
  6. Nov 24, 2014 #5

    Mark44

    Staff: Mentor

    If there's a way to solve this equation analytically, I don't see it. What I would do is try either a graphical approach, by graphing y = 4 * 5^2 - 3 * .4^(2x) and seeing where the graph crosses the line y = 11, or try an iterative approach, starting with an educated guess.

    If x = 1, the left side is 4 * 5^1 - 3 * .4^2 = 20 - .48 = 19.52
    If x = 2, the left side is slightly less than 100, so we're getting farther away from a solution, so try numbers smaller than 1.
     
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