Lesson Explainer: One-Variable Quadratic Inequalities

In this explainer, we will learn how to solve one-variable quadratic inequalities algebraically and graphically.

Recall that, in an equation, we have two expressions that are equal to each other, and we write the equals sign, , between them. When we have two expressions that are not equal to one another, we can relate the expressions by the use of an inequality sign.

We can have inequalities such as

In each of these inequalities, has a range of possible solutions. When we have an inequality such as , we can say this in words as “ is greater than or equal to four.” This means that a value of that is four or more will satisfy this inequality. The four inequality symbols we use are

We can solve inequalities in a similar process to solving equations, by ensuring that we perform the same mathematical operation to both sides of the inequality. However, as inequalities have a direction, we must be careful to consider which side of the inequality an expression is on. When we multiply or divide by a negative number, we must switch the inequality. For example, if we have then when dividing by we must switch the inequality to give

Let us now look at how to solve an inequality and represent the answer as an interval. Before doing so, we need to recap some notation. If we consider the interval of numbers from 0 to 10, which includes 0 but not 10, we could represent this using inequalities as

The strict inequality on the right tells us that the 10 is not included in the inequality, and the nonstrict inequality on the left tells us that the 0 is included. Another way of writing this interval would be .

Here, the closed square bracket tells us that the 0 is included, and the open square bracket tells us that the 10 is not included. It is also worth recapping here that the symbol for infinity is . This is often used to represent intervals that are greater than or less than a single number. For example, in interval notation would be .

In this explainer, we will be focusing on quadratic inequalities. This is in contrast to linear inequalities, which look something similar to the following:

Recall that the procedure to solve inequalities of this form is quite straightforward. The first thing we want to do is rearrange the inequality so all of the -terms are on one side and all the constant terms are on the other. We do this by subtracting 3 from both sides:

Then, to get this in terms of just , we divide each side by , remembering that when we divide an inequality by a negative number, we need to switch the inequality sign. This gives us

So, is all numbers greater than or equal to . This can also be expressed as an interval, as .

In the same way that we have distinct equations such as linear and quadratic equations, we can have quadratic inequalities in the following forms.

Definition: Quadratic Inequality

A quadratic inequality can be in one of the following forms: where , , and are constants and .

When we solve a quadratic inequality, we need to find the range of solutions, or intervals, for which an inequality is true. In comparison to the linear case, this is more difficult and can involve more than one distinct interval. We can solve quadratic inequalities using the process steps below.

How To: Solving a Quadratic Inequality Algebraically

Rearrange the inequality so that we have all the terms of the expression, defined as , on one side, with an inequality relating this to zero. For example, or .
Solve by factoring, or otherwise, to find the solutions to the equation.
Select test points for each interval so that there are values less than, between, and greater than the solutions of the equation. We can also use a sign chart to identify the intervals that will be positive or negative.
Identify the intervals that satisfy the inequality.

In the next example, we will look at how we can use a sign chart to identify positive and negative values for the intervals of an inequality.

Example 1: Solving a Quadratic Inequality Using a Sign Chart

Describe all solutions to the inequality .

Answer

To begin solving the inequality , we will first transform and rearrange this to get a positive coefficient of . We can multiply all terms of the coefficient by , recalling that when we multiply an inequality by a negative number, we must switch the inequality. This gives us

We now need to solve , where . We can factor our equation to give

Therefore,

To solve the inequality , we need to identify the regions where this is true. Whether or not depends on the signs of the factors and .

We can create a grid to identify whether each factor will be positive or negative in the intervals less than, greater than, and between our solutions of and . In the grid, we can place the intervals between our solutions horizontally and our factors of vertically, with the product of the factors below. We can then calculate whether the product of the factors will be positive or negative.

In the grid above, we can see in the first results column that when , our values and will both be negative, so the product of those two negative values, , will be positive.

Checking the sign of in the grid, we can see that it will be positive, that is, , when or . In other words, the inequality is satisfied when does not satisfy . In the interval notation, we can express our answer as .

In the first example, we were able to follow the process for solving a quadratic inequality exactly by first solving the quadratic and then using a sign chart, but we should be aware that this is not always what is desired or necessary. In the following example we will consider a quadratic inequality that cannot be factored.

Example 2: Solving a Quadratic Inequality

Find all solutions to the inequality . Write your answer as an interval.

Answer

As this inequality has been given to us with all terms on one side of the equation, no rearrangement will be necessary. Usually, to begin finding the solutions, we would try and solve , where . However, this actually has no solutions, since will always be greater than zero for any real value of . That is to say, we have

So, the left-hand side of the inequality will always be strictly greater than zero, meaning is never true.

Graphically, this can be seen by considering that the graph of never crosses the , as shown below.

Written as an interval, the solution is the empty set .

In the previous two examples, we solved quadratic inequalities where the right side of the inequality is equal to zero. When both sides of the inequality contain nonzero expressions, we need to first simplify the inequality to the point where one side of it is zero. In the next example, we will consider an example where both sides of the inequality contain quadratic expressions. After simplifying this inequality, we will solve it both algebraically and graphically.

Example 3: Solving a Quadratic Inequality

Determine the solution set of the inequality .

Answer

To begin solving this inequality, we will first simplify it so that one side is zero. Although it is tempting, we cannot simply take the square root of each side of the equality here. Since the square root can take both positive and negative values, taking a square root of an inequality can lead to an invalid answer. Instead, we can multiply through the parentheses in both sides of the inequality to obtain

We now need to collect all the terms on the same side of the inequality. In order to keep a positive coefficient of , we can subtract all the terms on the left-hand side from each side of the inequality, which gives us

Simplifying, we have

We can also write this as noting that the two sides of the inequality have been switched.

Since 24 is a common factor, we can divide all the terms by 24, which gives us

We have simplified the given inequality to the point where one side of it is zero. Now, we will finish solving this inequality using two different methods: the algebraic method and the graphical method.

Method 1

We first solve this inequality algebraically. Setting our and factoring give us

Therefore,

In factorized form, inequality (1) is written as , and we need to identify the regions where this inequality is true. Whether or not depends on the factors and .

We can create a grid to identify whether each factor will be positive or negative in the intervals less than, greater than, and between our solutions and . Since we have a nonstrict inequality, we can also record the values when and . We can then calculate whether the product of the factors will be positive or negative.


		0
				0
		0		0

From the grid, we can see that the intervals where are when and when . In other words, the inequality is satisfied when does not satisfy . In the interval notation, we can express our answer as .

Method 2

Let’s consider how to solve inequality (1) graphically. We begin by sketching a graph of . Given that the coefficient of is positive, we know that the parabola curve will open upward. In method 1, we have identified the roots of the equation to be and , this means that the curve will pass through the coordinates and .

To solve the inequality , we consider the points on the graph of , where . This will be above the , at the values where and where . As shown in method 1, our answer in interval notation can be written as .

In the previous example, we solved a quadratic inequality both algebraically and graphically. Both methods required simplifying the given inequality to the point where one side of it is zero. From there, it is simple to solve a quadratic inequality graphically as long as we can sketch the graph of the quadratic function. We will use the graphical method of solving quadratic inequalities in the remaining examples.

Let us consider another example of solving a quadratic inequality graphically.

Example 4: Solving a Quadratic Inequality Using a Graph

Solve the inequality .

Answer

We begin by simplifying the given inequality to the point where one side of it is zero. We can subtract from both sides, which gives us

We can then add 27 to both sides of the inequality, giving us

To solve the inequality graphically, we will sketch a graph of . To do this, we first need to find the points where the equation crosses the , often called the roots of the equation.

Setting our , we can factor this, giving us

So,

We now need to establish the shape of the curve . As the coefficient of , 2, is positive, this means that the parabola curve will open upward.

So, as the roots of the equation are and , we can plot the coordinates and and sketch a parabola curve as shown below.

Next, we need to identify the areas for which the inequality holds true. We can see from the sketch that is at values less than zero between the values and . Therefore, must satisfy . In interval notation, we can write this as .

We consider one more example of solving a quadratic inequality graphically.

Example 5: Solving a Quadratic Inequality Using a Graph

Solve the inequality .

Answer

We begin by simplifying the given inequality to the point where one side of it is zero. Multiplying through the parentheses, we obtain

To solve the inequality graphically, we will sketch a graph of . To do this, we first need to find the roots of the quadratic function . These roots can be found by setting and solving, giving us

Factoring, we have

Therefore,

As the coefficient of in the equation is 1, this value is greater than zero; so the parabola curve will open upward. As the roots of the equation are and , this means that the curve will pass through the coordinates and . We can sketch the graph shown below.

To solve the inequality , we consider the points on the graph of , where . This will be above the at the values where is less than 0 and where is greater than 7. Because we do not have a strict inequality, can also be exactly equal to 0 or 7. Another way to express this would be by saying that is all the values excluding the points where . We can express this final answer in interval notation as

Let’s finish by recapping a few important concepts from the explainer.

Key Points

The solution of a quadratic inequality is an interval or a union of intervals. When it is a union of two intervals, we can use the set difference notation to write it as the complement of one interval.
To solve a quadratic inequality algebraically, we follow the steps below:
- Rearrange the inequality so that we have all the terms of the expression on one side, with an inequality relating this to zero, for example, .
- Factor the inequality by setting to identify the roots of the expression .
- Identify the intervals that satisfy the inequality by using test points in each interval or a sign chart. We can also sketch a graph of the function.
To solve a quadratic inequality graphically, we follow the steps below.
- Rearrange the inequality so that we have all the terms of the expression on one side, with an inequality relating this to zero; for example .
- Factor the inequality by setting , to identify the roots of the expression .
- Sketch the graph of the equation , using the roots of the equation and finding the direction of the parabola curve. Take special care if you have rearranged the original inequality to change the sign of the value: use the coefficient in the rearranged form of the inequality to identify the shape of the curve, rather than the original inequality.
- Identify the intervals that satisfy the inequality.