Incorrectly simplifying expressions inside brackets

Oct 13, 2025 by wbrum.org 142 views

Introduction

Logarithmic equations are an essential part of mathematics, particularly in calculus and algebra. They are used to solve problems involving exponential growth and decay, and are a crucial tool in many fields, including physics, engineering, and economics. However, solving logarithmic equations can be challenging, and students often make mistakes when attempting to solve them. In this article, we will examine a student solution to a logarithmic equation and identify the error in their work.

The Student Solution

The student solution is as follows:

{ \begin{array}{c} 2 \ln (x)=\ln (3 x)-[\ln (9)-2 \ln (3)] \\ \ln \left(x^2\right)=\ln (3 x)-[\ln (9)-\ln (9)] \\ \ln \left(x^2\right)=\ln (3 x)-0 \\ \ln (x^2)=\ln (3x) \\ x^2=3x \\ x^2-3x=0 \\ x(x-3)=0 \\ x=0 \text{ or } x=3 \end{array} }

Identifying the Error

At first glance, the student solution appears to be correct. However, upon closer inspection, we can see that there is an error in the solution. The error occurs in the second step, where the student simplifies the expression inside the brackets.

{ \ln \left(x^2\right)=\ln (3 x)-[\ln (9)-\ln (9)] }

The student incorrectly simplifies the expression inside the brackets to zero, when in fact, it should be simplified to $\ln (9)$ .

{ \ln \left(x^2\right)=\ln (3 x)-\ln (9) }

This error has a ripple effect throughout the rest of the solution, leading to incorrect conclusions.

Correcting the Error

To correct the error, we need to revisit the second step and simplify the expression inside the brackets correctly.

{ \begin{array}{c} 2 \ln (x)=\ln (3 x)-[\ln (9)-2 \ln (3)] \\ \ln \left(x^2\right)=\ln (3 x)-[\ln (9)-\ln (9)] \\ \ln \left(x^2\right)=\ln (3 x)-\ln (9) \\ \ln (x^2)=\ln (3x)-\ln (9) \\ \ln (x^2)-\ln (3x)=-\ln (9) \\ \ln \left(\frac{x^2}{3x}\right)=-\ln (9) \\ \ln \left(\frac{x}{3}\right)=-\ln (9) \\ \ln \left(\frac{x}{3}\right)=\ln \left(\frac{1}{9}\right) \\ \frac{x}{3}=\frac{1}{9} \\ x=\frac{1}{27} \end{array} }

Conclusion

In conclusion, the student solution to the logarithmic equation contains an error in the second step, where the expression inside the brackets is incorrectly simplified to zero. This error has a ripple effect throughout the rest of the solution, leading to incorrect conclusions. By correcting the error and simplifying the expression inside the brackets correctly, we arrive at the correct solution to the logarithmic equation.

Common Mistakes to Avoid

When solving logarithmic equations, there are several common mistakes to avoid. These include:

Incorrectly simplifying expressions inside brackets: This is the error that occurred in the student solution. It is essential to simplify expressions inside brackets correctly to avoid incorrect conclusions.
Failing to use the properties of logarithms: Logarithmic equations rely heavily on the properties of logarithms. Failing to use these properties can lead to incorrect solutions.
Not checking the domain of the logarithmic function: Logarithmic functions have a restricted domain. Failing to check the domain of the logarithmic function can lead to incorrect solutions.

Tips for Solving Logarithmic Equations

Solving logarithmic equations can be challenging, but there are several tips that can help. These include:

Using the properties of logarithms: Logarithmic equations rely heavily on the properties of logarithms. Using these properties can help simplify the equation and arrive at the correct solution.
Checking the domain of the logarithmic function: Logarithmic functions have a restricted domain. Checking the domain of the logarithmic function can help ensure that the solution is correct.
Using algebraic techniques: Algebraic techniques, such as factoring and canceling, can help simplify the equation and arrive at the correct solution.

Conclusion

Introduction

Logarithmic equations are a fundamental concept in mathematics, and solving them can be challenging. In our previous article, we examined a student solution to a logarithmic equation and identified the error in their work. In this article, we will provide a Q&A guide to help you better understand logarithmic equations and how to solve them.

Q: What is a logarithmic equation?

A: A logarithmic equation is an equation that involves a logarithmic function. Logarithmic functions are used to solve problems involving exponential growth and decay.

Q: What are the properties of logarithms?

A: The properties of logarithms are:

Product rule: $\log_b (xy) = \log_b x + \log_b y$
Quotient rule: $\log_b \left(\frac{x}{y}\right) = \log_b x - \log_b y$
Power rule: $\log_b (x^n) = n \log_b x$
Change of base rule: $\log_b x = \frac{\log_a x}{\log_a b}$

Q: How do I simplify a logarithmic expression?

A: To simplify a logarithmic expression, you can use the properties of logarithms. For example, if you have the expression $\log_b (xy)$ , you can use the product rule to simplify it to $\log_b x + \log_b y$ .

Q: How do I solve a logarithmic equation?

A: To solve a logarithmic equation, you can use the properties of logarithms to simplify the equation and then isolate the variable. For example, if you have the equation $\log_b x = 2$ , you can use the power rule to simplify it to $x = b^2$ .

Q: What are some common mistakes to avoid when solving logarithmic equations?

A: Some common mistakes to avoid when solving logarithmic equations include:

Incorrectly simplifying expressions inside brackets: This is the error that occurred in the student solution. It is essential to simplify expressions inside brackets correctly to avoid incorrect conclusions.
Failing to use the properties of logarithms: Logarithmic equations rely heavily on the properties of logarithms. Failing to use these properties can lead to incorrect solutions.
Not checking the domain of the logarithmic function: Logarithmic functions have a restricted domain. Failing to check the domain of the logarithmic function can lead to incorrect solutions.

Q: How do I check the domain of a logarithmic function?

A: To check the domain of a logarithmic function, you need to ensure that the argument of the logarithm is positive. For example, if you have the function $\log_b x$ , you need to ensure that $x > 0$ .

Q: What are some tips for solving logarithmic equations?

A: Some tips for solving logarithmic equations include:

Using the properties of logarithms: Logarithmic equations rely heavily on the properties of logarithms. Using these properties can help simplify the equation and arrive at the correct solution.
Checking the domain of the logarithmic function: Logarithmic functions have a restricted domain. Checking the domain of the logarithmic function can help ensure that the solution is correct.
Using algebraic techniques: Algebraic techniques, such as factoring and canceling, can help simplify the equation and arrive at the correct solution.

Conclusion

In conclusion, solving logarithmic equations requires careful attention to detail and a thorough understanding of the properties of logarithms. By avoiding common mistakes and using algebraic techniques, we can arrive at the correct solution to logarithmic equations. We hope this Q&A guide has been helpful in providing you with a better understanding of logarithmic equations and how to solve them.