SOLUTION: Solve for x if log x base5+6 log5 base x=5

Algebra ->  Logarithm Solvers, Trainers and Word Problems -> SOLUTION: Solve for x if log x base5+6 log5 base x=5      Log On


   



Question 1207274: Solve for x if log x base5+6 log5 base x=5
Answer by ikleyn(52909) About Me  (Show Source):
You can put this solution on YOUR website!
.
Solve for x if log x base5+6 log5 base x=5
~~~~~~~~~~~~~~~~~~

The starting equation is

    log%285%2Cx%29 + 6%2Alog%28x%2C5%29 = 5.


Change the base  log%28x%2C5%29 = 1%2Flog%285%2Cx%29.


Then the original equation takes the form

    log%285%2Cx%29 + 6%2Flog%285%2Cx%29 = 5.


Introduce new variable y = log%285%2Cx%29.
Then the equation takes the form

    y + 6%2Fy= 5.


Multiply all the term by y and reduce to quadratic equation

    y^2 - 5y + 6 = 0.


Factor left side

    (y-2)*(y-3) = 0.


The roots are y= 2 and y= 3.


From the root y= 2 we get  log%285%2Cx%29 = 2,  x = 5%5E2 = 25.

From the root y= 3 we get  log%285%2Cx%29 = 3,  x = 5%5E3 = 125.


At this point, the problem is just solved in full.


ANSWER.  The given equation has two solutions, x= 25  and  x= 125.

Solved.

---------------

On logarithms and their properties,  see these introductory lessons
    - WHAT IS the logarithm
    - Properties of the logarithm
    - Change of Base Formula for logarithms
    - Evaluate logarithms without using a calculator
    - Simplifying expressions with logarithms
    - Solving logarithmic equations
in this site.