Solve Logarithmic Equation: 3 Solutions Explained

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SUMMARY

The logarithmic equation (ln x)^3 = 3 ln x has three solutions: x = 1, x = e^sqrt(3), and x = e^-sqrt(3). The discussion highlights that a common mistake is dividing by ln x, which can lead to the loss of potential solutions. The substitution y = ln x simplifies the equation to y^3 - 3y = 0, allowing for the identification of all three solutions. Understanding the implications of squaring both sides of the equation is crucial to avoid missing solutions.

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Students studying algebra, particularly those focusing on logarithmic functions, and educators looking for effective methods to teach solving logarithmic equations.

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Homework Statement


I'm trying to solve this logarithmic equation, which has 3 possible solutions. However, I can only get to one of them. What are other ways I can solve it so I get to these other solutions?


Homework Equations


(ln x)^3 = 3 ln x


The Attempt at a Solution


(ln x)^3 = 3 ln x
(ln x)^2 = 3
ln x = sqrt(3)
e^sqrt(3) = x
 
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MAke the substitution y=lnx. Then your equation becomes y3-3y=0. Can you solve this for y?

(Note: You don't have to make the substitution; you could do it using your method. Your problems are:
(ln x)^3 = 3 ln x
(ln x)^2 = 3
Here you lost a solution by dividing by lnx
ln x = sqrt(3)
Here you only took one value of the square root)
 
Last edited:
Thanks a lot.

x = 1, e^sqrt(3), e^-sqrt(3)
 

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