Skip to ContentGo to accessibility pageKeyboard shortcuts menu
OpenStax Logo
Algebra 1

9.7.2 Common Calculation Errors When Using the Quadratic Formula

Algebra 19.7.2 Common Calculation Errors When Using the Quadratic Formula

Search for key terms or text.

Activity

Here are four equations, followed by attempts to solve them using the quadratic formula. Each attempt contains at least one error.

Solve the equations by using the quadratic formula. Then, find and describe the error(s) in the worked solutions of the same equations as the ones you solved.

1.

Equation 1: 2x2+3=8x2x2+3=8x

Here is the worked solution with errors:

Step 1 - 2x28x+3=02x28x+3=0

Step 2 - a=2a=2, b=8b=8, c=3c=3

Step 3 - x=b±b24ac2ax=b±b24ac2a

Step 4 - x=(8)±(8)2(4)(2)(3)2(2)x=(8)±(8)2(4)(2)(3)2(2)

Step 5 - x=8±64244x=8±64244

Step 6 - x=8±404x=8±404

Step 7 - x=2±10x=2±10

2.

Equation 2: x2+3x=10x2+3x=10

Here is the worked solution with errors:

Step 1 - a=1a=1, b=3b=3, c=10c=10

Step 2 - x=b±b24ac2ax=b±b24ac2a

Step 3 - x=(3)±32(4)(1)(10)2(1)x=(3)±32(4)(1)(10)2(1)

Step 4 - x=3±9402x=3±9402

Step 5 - x=3±312x=3±312

No Solutions

3.

Equation 3: 9x22x1=09x22x1=0

Here is the worked solution with errors:

Step 1 - a=9a=9, b=2b=2, c=1c=1

Step 2 - x=b±b24ac2ax=b±b24ac2a

Step 3 - x=2±(2)2(4)(9)(1)2x=2±(2)2(4)(9)(1)2

Step 4 - x=2±4+362x=2±4+362

Step 5 - x=2±402x=2±402

4.

Equation 4: x210x+23=0x210x+23=0

Here is the worked solution with errors:

Step 1 - a=1a=1, b=10b=10, c=23c=23

Step 2 - x=b±b24ac2ax=b±b24ac2a

Step 3 - x=10±(10)2(4)(1)(23)2x=10±(10)2(4)(1)(23)2

Step 4 - x=10±100922x=10±100922

Step 5 - x=10±1922x=10±1922

No Solutions

Self Check

When solving x 2 30 x + 26 = 0 using the quadratic formula, Sloane's work showed the formula as this:

x = 30 ± 900 104 2 .

Which of the following describes the error in her work?

  1. The discriminant should be 900 104 , not 900 104 ; x = 30 ± 900 104 2 .
  2. The first term in the numerator should be 30, not -30; x = 30 ± 900 104 2 .
  3. The discriminant should be 900 + 104 , not 900 104 ; x = 30 ± 900 + 104 2 .
  4. The denominator should be 4, not 2; x = 30 ± 900 104 4 .

Additional Resources

Common Errors When Using the Quadratic Formula

The quadratic formula has many parts in it. A small error in any one part can lead to incorrect solutions.

Suppose we are solving 2x26=11x2x26=11x. To use the formula, let's rewrite it in the form of ax2+bx+c=0ax2+bx+c=0, which gives: 2x211x6=02x211x6=0.

Here are some common errors to avoid:

Using the wrong values for aa, bb, and cc in the formula.

x=b±b24ac2ax=b±b24ac2a

x=11±(11)24(2)(6)2(2)x=11±(11)24(2)(6)2(2)

This is inbb is -11, so bb is -(-11), which is 11, not -11.

The correct formula is:
x=11±(11)24(2)(6)2(2)x=11±(11)24(2)(6)2(2)

Forgetting to multiply 2 by aa for the denominator in the formula.

x=11±(11)24(2)(6)2x=11±(11)24(2)(6)2

This is inThe denominator is 2a2a, which is 2(2)2(2) or 4.

The correct formula is:
x=11±(11)24(2)(6)2(2)x=11±(11)24(2)(6)2(2)

Forgetting that squaring a negative number produces a positive number.

x=11±1214(2)(6)4x=11±1214(2)(6)4

This is in(11)2(11)2 is 121, not -121.

The correct formula is:
x=11±1214(2)(6)4x=11±1214(2)(6)4

Forgetting that a negative number times a positive number is a negative number.

x=11±121484x=11±121484

This is in4(2)(6)=484(2)(6)=48 and 121(48)121(48) is 121+48121+48.

The correct formula is:
x=11±121+484x=11±121+484

Making calculation errors or not following the properties of algebra.

x=11±1694x=11±1694

x=11±42.25x=11±42.25

This is inBoth parts of the numerator, the 11 and the 169169, get divided by 4. Also, 16941694 is not 42.2542.25.

The correct formula is:
x=11±134x=11±134

Let's finish by evaluating 11±13411±134 correctly:
x=11+134x=11+134 or x=11134x=11134
x=244x=244 or x=24x=24
x=6x=6 or x=12x=12

To make sure our solutions are indeed correct, we can substitute the solutions back into the original equations and see whether each solution keeps the equation true.

Checking 6 as a solution:
2x26=11x2x26=11x
2(6)26=11(6)2(6)26=11(6)
2(36)6=662(36)6=66
726=66726=66 66=6666=66

Checking 1212 as a solution:
2x26=11x2x26=11x
2(12)26=11(12)2(12)26=11(12)
2(14)6=1122(14)6=112
126=512126=512
512=512512=512

We can also graph the equation y=2x211x6y=2x211x6 and find its xx-intercepts to see whether our solutions to 2x211x6=02x211x6=0 are accurate (or close to accurate).

Graph of a parabola on a coordinate plane. Two points (negative one-half, 0) and (6, 0) have been labeled on the parabola. The x-axis has a scale of 1 extending from negative 3 to 8. The y-axis extends from negative 24 to 8 with a scale of 4.

Try it

Try It: Common Errors When Using the Quadratic Formula

Zariah was solving the equation 3x28x+4=03x28x+4=0 with the quadratic formula and used the steps below. She then had to stop because she realized there was a problem. Locate and describe her error.

Step 1 - x=(8)±824(3)(4)6x=(8)±824(3)(4)6

Step 2 - x=8±64486x=8±64486

Citation/Attribution

This book may not be used in the training of large language models or otherwise be ingested into large language models or generative AI offerings without OpenStax's permission.

Want to cite, share, or modify this book? This book uses the Creative Commons Attribution-NonCommercial-ShareAlike License and you must attribute OpenStax.

Attribution information
  • If you are redistributing all or part of this book in a print format, then you must include on every physical page the following attribution:

    Access for free at https://openstax.org/books/algebra-1/pages/about-this-course

  • If you are redistributing all or part of this book in a digital format, then you must include on every digital page view the following attribution:

    Access for free at https://openstax.org/books/algebra-1/pages/about-this-course

Citation information

© May 21, 2025 OpenStax. Textbook content produced by OpenStax is licensed under a Creative Commons Attribution-NonCommercial-ShareAlike License . The OpenStax name, OpenStax logo, OpenStax book covers, OpenStax CNX name, and OpenStax CNX logo are not subject to the Creative Commons license and may not be reproduced without the prior and express written consent of Rice University.