The Importance of Checking the Domain

Oct 13, 2025 by wbrum.org 68 views

Introduction

In mathematics, solving equations is a fundamental concept that requires careful analysis and attention to detail. Recently, a student claimed to have found two solutions to the equation $\sqrt{3x+7}=x-1$ . The student's proposed solutions were $x=-1$ and $x=6$ . In this article, we will examine the student's solution and provide a critical analysis of their work.

The Student's Solution

The student's solution to the equation $\sqrt{3x+7}=x-1$ is as follows:

Square both sides of the equation: $\sqrt{3x+7}^2=(x-1)^2$
Simplify the equation: $3x+7=x^2-2x+1$
Rearrange the equation: $x^2-5x-6=0$
Factor the quadratic equation: $(x-6)(x+1)=0$
Solve for x: $x=6$ or $x=-1$

A Critical Analysis of the Student's Solution

While the student's solution appears to be correct, there is a crucial step that was overlooked. When squaring both sides of the equation, the student failed to consider the possibility that the expression inside the square root may be negative. This is a critical error that can lead to incorrect solutions.

The Importance of Checking the Domain

When working with square roots, it is essential to check the domain of the expression inside the square root. In this case, the expression inside the square root is $3x+7$ . To find the domain, we need to determine when $3x+7 \geq 0$ .

Solving the Inequality

To solve the inequality $3x+7 \geq 0$ , we can subtract 7 from both sides:

$3x \geq -7$

Next, we can divide both sides by 3:

$x \geq -\frac{7}{3}$

The Domain of the Expression

The domain of the expression inside the square root is $x \geq -\frac{7}{3}$ . This means that the expression inside the square root is always non-negative when $x \geq -\frac{7}{3}$ .

The Implication of the Domain

The domain of the expression inside the square root has a significant implication for the student's solution. Since the expression inside the square root is always non-negative when $x \geq -\frac{7}{3}$ , the square root is defined for all values of x in this domain. However, when $x < -\frac{7}{3}$ , the expression inside the square root is negative, and the square root is undefined.

The Correct Solution

Given the domain of the expression inside the square root, we can re-examine the student's solution. The student's proposed solutions were $x=-1$ and $x=6$ . However, since $x=-1$ is less than $-\frac{7}{3}$ , the expression inside the square root is negative, and the square root is undefined. Therefore, $x=-1$ is not a valid solution.

Conclusion

In conclusion, while the student's solution appears to be correct, there is a crucial step that was overlooked. The student failed to consider the possibility that the expression inside the square root may be negative, which led to an incorrect solution. By checking the domain of the expression inside the square root, we can determine that the student's proposed solution $x=-1$ is not valid. Therefore, we do not agree with the student's solution.

Recommendations for Future Improvement

To improve their solution, the student should have:

Checked the domain of the expression inside the square root.
Considered the possibility that the expression inside the square root may be negative.
Verified that the proposed solutions satisfy the original equation.

By following these recommendations, the student can improve their solution and provide a more accurate answer.

Final Thoughts

Solving equations is a critical skill in mathematics, and it requires careful attention to detail. By checking the domain of the expression inside the square root and considering the possibility that the expression may be negative, we can ensure that our solutions are accurate and valid. In this article, we have demonstrated the importance of checking the domain and provided a critical analysis of the student's solution. We hope that this article will serve as a valuable resource for students and educators alike.

Additional Resources

References

About the Author

Introduction

In our previous article, we discussed the importance of checking the domain of the expression inside the square root when solving equations with square roots. We also provided a critical analysis of a student's solution to the equation $\sqrt{3x+7}=x-1$ . In this article, we will provide a Q&A section to help students better understand the concept of solving equations with square roots.

Q: What is the domain of the expression inside the square root?

A: The domain of the expression inside the square root is the set of all values of x for which the expression is non-negative. In other words, the domain is the set of all values of x that make the expression inside the square root greater than or equal to zero.

Q: How do I check the domain of the expression inside the square root?

A: To check the domain of the expression inside the square root, you need to determine when the expression is non-negative. This can be done by setting the expression greater than or equal to zero and solving for x.

Q: What happens if the expression inside the square root is negative?

A: If the expression inside the square root is negative, the square root is undefined. This means that the equation has no solution for x.

Q: How do I solve equations with square roots?

A: To solve equations with square roots, you need to follow these steps:

Check the domain of the expression inside the square root.
Square both sides of the equation.
Simplify the equation.
Solve for x.

Q: What if I get a negative value under the square root?

A: If you get a negative value under the square root, it means that the expression inside the square root is negative. In this case, the square root is undefined, and the equation has no solution for x.

Q: Can I use the quadratic formula to solve equations with square roots?

A: Yes, you can use the quadratic formula to solve equations with square roots. However, you need to be careful when using the quadratic formula, as it may not always give you the correct solution.

Q: How do I know if my solution is valid?

A: To know if your solution is valid, you need to check that it satisfies the original equation. This means that you need to plug your solution back into the original equation and check that it is true.

Q: What if I get multiple solutions to the equation?

A: If you get multiple solutions to the equation, it means that the equation has multiple solutions. However, you need to be careful when checking the validity of each solution, as some solutions may be extraneous.

Conclusion

Solving equations with square roots can be challenging, but with practice and patience, you can master this skill. Remember to always check the domain of the expression inside the square root and to be careful when using the quadratic formula. By following these tips, you can ensure that your solutions are accurate and valid.

Additional Resources

References

About the Author

The author is a mathematics educator with over 10 years of experience teaching algebra and trigonometry. They have a passion for making mathematics accessible and enjoyable for students of all ages.