Quadratic inequalities
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Quadratic Inequalities. Tidewater Community College Karen Overman. Quadratics. Before we get started let’s review. A quadratic equation is an equation that can be written in the form , where a, b and c are real numbers and a cannot equal zero.

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Quadratic inequalities l.jpg

Quadratic Inequalities

Tidewater Community College

Karen Overman


Quadratics l.jpg
Quadratics

Before we get started let’s review.

A quadratic equation is an equation that can

be written in the form ,

where a, b and c are real numbers and a cannot equal

zero.

In this lesson we are going to discuss quadratic

inequalities.


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Quadratic Inequalities

What do they look like?

Here are some examples:


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Quadratic Inequalities

When solving inequalities we are trying to

find all possible values of the variable

which will make the inequality true.

Consider the inequality

We are trying to find all the values of x for which the

quadratic is greater than zero or positive.


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Solving a quadratic inequality

We can find the values where the quadratic equals zero

by solving the equation,


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Solving a quadratic inequality

You may recall the graph of a quadratic function is a parabola and the values we just found are the zeros or x-intercepts.

The graph of is


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Solving a quadratic inequality

From the graph we can see that in the intervals around the zeros, the graph is either above the x-axis (positive) or below the x-axis (negative). So we can see from the graph the interval or intervals where the inequality is positive. But how can we find this out without graphing the quadratic?

We can simply test the intervals around the zeros in the quadratic inequality and determine which make the inequality true.


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Solving a quadratic inequality

For the quadratic inequality,

we found zeros 3 and –2 by solving the equation

. Put these values on a number line and we can see three intervals that we will test in the inequality. We will test one value from each interval.

-2

3



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Solving a quadratic inequality

Thus the intervals make up the solution set for the quadratic inequality, .

In summary, one way to solve quadratic inequalities is to find the zeros and test a value from each of the intervals surrounding the zeros to determine which intervals make the inequality true.


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Example 2:

Solve

First find the zeros by solving the equation,


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Example 2:

Now consider the intervals around the zeros and test a value from each interval in the inequality.

The intervals can be seen by putting the zeros on a number line.

1/2

1



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Example 2:

Thus the interval makes up the solution set for

the inequality .


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Example 3:

Solve the inequality .

First find the zeros.


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Example 3:

But these zeros , are complex numbers.

What does this mean?

Let’s look at the graph of the quadratic,


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Example 3:

We can see from the graph of the quadratic that the curve never intersects the x-axis and the parabola is entirely below the x-axis. Thus the inequality is always true.


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Example 3:

How would you get the answer without the graph?

The complex zeros tell us that there are no REAL zeros, so the parabola is entirely above or below the x-axis. At this point you can test any number in the inequality, If it is true, then the inequality is always true. If it is false, then the inequality is always false.

We can also determine whether the parabola opens up or down by the leading coefficient and this will tell us if the parabola is above or below the x-axis.


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Summary

In general, when solving quadratic inequalities

  • Find the zeros by solving the equation you get when you replace the inequality symbol with an equals.

  • Find the intervals around the zeros using a number line and test a value from each interval in the number line.

  • The solution is the interval or intervals which make the inequality true.



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