Christmas Streak 35/88: Seems Easy Enough!

Algebra Level 5

How many real values for x x are there that satisfy the below equation?

log x 1 1 6 + log x 1 ( x + 4 ) + log x 1 ( x 3 + 7 x 2 13 x + 9 ) = 0 \large \log_{x-1} \frac{1}{6}+\log_{x-1}(-x+4)+\log_{x-1}(-x^3+7x^2-13x+9)=0


Details:

  • We are taking the real logarithm defined only in reals.

This problem is a part of <Christmas Streak 2017> series .

2 0 1 4 3

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1 solution

Boi (보이)
Nov 3, 2017

Relevant wiki: Properties of Logarithms - Basic

Using the basic rules of logarithms, we can simplify the equation to log x 1 { 1 6 ( x + 4 ) ( x 3 + 7 x 2 13 x + 9 ) } = 0. \displaystyle \log_{x-1} \left\{\dfrac{1}{6}(-x+4)(-x^3+7x^2-13x+9)\right\}=0.

Since the antilogarithm must equal 1, we can write ( x 4 ) ( x 3 7 x 2 + 13 x 9 ) = 6. (x-4)(x^3-7x^2+13x-9)=6.

Simplifying this, we get ( x 1 ) ( x 2 ) ( x 3 ) ( x 5 ) = 0. (x-1)(x-2)(x-3)(x-5)=0.

Then are there 4 values for x x ?


Woah, wait, no, hold on there for a sec. Calm down.

We're taking the real logarithm. Check if the base is positive and not equal to 1, and also if the antilogarithm isn't negative.

If x = 1 , x=1, the base is equal to 0. 0. Then all the logs aren't defined.

If x = 2 , x=2, the base is equal to 1. 1. Similarly, all the logs aren't defined.

If x = 3 , x=3, the equation would look like: log 2 1 6 + log 2 1 + log 2 6 = 0. \displaystyle \log_{2} \frac{1}{6}+\log_{2}1+\log_{2}6=0. Makes perfect sense.

If x = 5 , x=5, the second and third antilogarithm is negative. Then those two logs aren't defined.


From above, we can figure out that there is only 1 value for x , x, which is 3. 3.

(P.S. If you type this up in wolframalpha, it'll tell you x = 1 or 3. Defeating the computer is easy. >w>)

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