Copyright

Discriminant: Definition & Explanation

Instructor: Yuanxin (Amy) Yang Alcocer

Amy has a master's degree in secondary education and has taught math at a public charter high school.

The discriminant helps you to solve quadratic equations. In this lesson, you will learn how to identify the discriminant and how to use it to help you find the number of solutions in a quadratic equation.

Definition

The discriminant is easy to find when you look at the quadratic formula. The quadratic formula is the equation you use to find the solutions to quadratic equations. It looks like this:

This is the quadratic formula. Look for the square root symbol - the contents inside are the discriminant.
Quadratic formula

The discriminant is the part inside the square root. Take a moment to look for the square root and find what is inside the square root. Once you see it, you will have found the discriminant. If we isolate that part, we get the formula for finding the discriminant, which is this:

This is the formula for finding the discriminant.
Discriminant

Using the Discriminant

The discriminant tells you how many possible solutions a particular quadratic equation has. Before we can use the quadratic equation, though, we first have to change it to standard form. Standard form is when all the variables and constants are on one side of the equation, and the other side is a zero. It looks like this:

This is a quadratic equation in standard form.
standard form of quadratic equation

Once you have your quadratic equation in standard form, you can label your numbers with the appropriate letters and plug the values into your formula for finding the discriminant. The result of your discriminant tells you how many solutions your quadratic has.

Examples

Let's look at an example:

Example 1
positive discriminant example

In our example, our quadratic equation gives us 1 for our letter a, 5 for letter b, and 4 for letter c. We take these values and plug them into their appropriate places in the discriminant formula, and we'll find that our discriminant equals 9, a positive number. This tells us that our quadratic equation has two possible real solutions. Real solutions are solutions that can be calculated using the quadratic formula. When you graph this quadratic equation, you will see that the curve crosses the x-axis in two places, exactly where your solutions are.

While the discriminant tells us the number of possible solutions, it does not tell us what those solutions are. But, it does give us a clue as to how many solutions we need to look for.

Remember that, when there are no numbers in front of variables, it is assumed that there is a 1 in front. We don't write the 1 because it is a mathematical convention and because it looks neater, especially when you have a lot of letters to work with.

Let's look at another example:

Example 2
negative discriminant example

To unlock this lesson you must be a Study.com Member.
Create your account

Register to view this lesson

Are you a student or a teacher?

Unlock Your Education

See for yourself why 30 million people use Study.com

Become a Study.com member and start learning now.
Become a Member  Back
What teachers are saying about Study.com
Try it risk-free for 30 days

Earning College Credit

Did you know… We have over 160 college courses that prepare you to earn credit by exam that is accepted by over 1,500 colleges and universities. You can test out of the first two years of college and save thousands off your degree. Anyone can earn credit-by-exam regardless of age or education level.

To learn more, visit our Earning Credit Page

Transferring credit to the school of your choice

Not sure what college you want to attend yet? Study.com has thousands of articles about every imaginable degree, area of study and career path that can help you find the school that's right for you.

Create an account to start this course today
Try it risk-free for 30 days!
Create An Account
Support