SOLUTION: Solve for m in (1/27)^m+81^-m=243

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Question 1176011: Solve for m in (1/27)^m+81^-m=243
Found 2 solutions by CubeyThePenguin, ikleyn:
Answer by CubeyThePenguin(3113) About Me  (Show Source):
You can put this solution on YOUR website!
3^(-3m) + 3^(-4m) = 3^5

-3m - 4m = 5

m = -5/7

Answer by ikleyn(52781) About Me  (Show Source):
You can put this solution on YOUR website!
.


            The  "solution"  in the post by @CubeyThePenguin is  TOTALLY  WRONG.

            It has nothing in common with the correct solution.


Introduce new variable  x = %281%2F3%29%5Em.


Then the equation takes the form


    x^4 + x^3 - 243 = 0.



The roots of the polynomial  (approximate values)  are:


    x%5B1%5D =  3.72008   

    x%5B2%5D = −4.22409

    x%5B3%5D = −0.248 + 3.9246i

    x%5B4%5D = −0.248 − 3.9246i



This polynomial has no rational roots that can be found using Rational Root Test.

Roots were found using quartic formulas.



Of these roots for x = %281%2F3%29%5Em,  for us only the positive real value 

    x%5B1%5D =  3.72008

make sense.


Then from  %281%2F3%29%5Em = 3.72008   you have  3%5Em = 1%2F3.72008 = 0.26881.


And THEREFORE,  m*log(3) = log(0.26881), which implies


    m = log%28%280.26881%29%29%2Flog%28%283%29%29 = -1.19583   (approximate).    ANSWER

Solved.