# Lesson Explainer: Introduction to Inequalities in Triangles Mathematics

In this explainer, we will learn how to define the concept of inequality in general and the axioms of inequality and compare between the measures of angles given in a triangle.

Before we begin discussing inequalities in triangles, we first need to recall what inequalities represent and how we can manipulate them. First, we recall that we use the symbol < to represent “less than” and > to represent “greater than.” For example, we can say that since 5 is a larger value than 3.

We can use inequalities to represent a comparison between any two numbers. This is particularly useful if the numbers represent a quantity such as distance. To see why this is useful, consider the following diagram:

We can see that the line segment is longer than the line segment , but writing this sentence out in full is not as succinct as writing the inequality .

This extends to any quantities represented by numbers. For example, consider the following two angles:

We can see in the diagram that the second angle is larger than the first angle. However, we need to be slightly careful. Remember that we describe the measure of the angle by a number and not the actual angle itself. So, when we are comparing the sizes of the angles, we should be comparing their measures. We can write this as .

We also recall that we can manipulate these inequalities to find equivalent inequalities. This can be useful in solving algebraic inequalities, such as finding all the values of that satisfy the inequality . To do this, we subtract 1 from both sides of the inequality to get . However, this can also be useful in comparing lengths, angle measures, or any other quantities.

For example, consider the following angles.

We see these two angles together make up less than a straight angle, and we know that the measure of a straight angle is . Therefore, we can conclude using the diagram that the sum of the measures of these angles is less than . We write this as .

To manipulate inequalities, we apply the same operation to both sides of the inequality using rules called the axioms of inequalities. We define the axioms of inequalities as follows:

### Definition: Axioms of Inequalities

For any real numbers , , , and , we have the following:

• We can compare the sizes of any two real numbers. In other words, either ,, or .
• Inequalities and equalities are transitive relations on the set of real numbers. This means that if and , then we must have that . This is called the transitivity axiom.
• Adding a constant to or subtracting a constant from both sides of an inequality gives an equivalent inequality. For example, if , then and .
• Multiplying or dividing both sides of an inequality by a positive constant yields an equivalent inequality. For example, if and , then and .
• We can combine the ideas of the transitivity axiom with the other axioms. For example, if and , then .

We can apply these axioms to any inequality to help solve inequalities or rewrite inequalities in a way that highlights certain relationships. For example, we can recall that the sum of the measures of the interior angles in a triangle is . If we consider a right triangle , we can prove that the nonright angles must be acute.

First, we note that . We subtract from both sides to get

We know that we can always compare the sizes of real numbers, so we can compare to and to . Since their measures add to and they are both positive, neither angle can have a measure greater than or equal to . Hence,

Let’s now see some examples of comparing geometric quantities using inequalities and using the axioms of inequalities to prove geometric relationships.

### Example 1: Forming Inequalities in a Triangle

Look at the figure.

Use <, =, or > to fill in the blank in the following.

• .
• .
• .
• .

To compare the measures of these angles, we will start by finding the measures of the angles in the diagram. We can do this by recalling two facts:

• The sum of the measures of the internal angles in a triangle is .
• The sum of the measures of the angles that make up a straight angle is .

We can find by noting is an internal angle of triangle . Thus,

Next, we can find by noting combines with to make a straight angle. Therefore,

Substituting into the equation and solving yields

We now have the measures of two of the internal angles in triangle , so we can determine the measure of the final internal angle. We have

Finally, we see that combines with to make a straight angle, so their measures sum to . Thus,

We can add these angle measures onto the diagram to get the following:

We can now compare the measures of all of these angles.

Firstly, and , so .

Secondly, and , so .

Thirdly, and , so .

Finally, and , so .

In the previous example, we compared the measures of the interior and exterior angles in a triangle by using their measures, the measure of a straight angle, and the sum of the measures of the interior angles in a triangle.

It is worth noting we can use this same reasoning to show a useful inequality that holds in general. Consider any triangle and any of its exterior angles, for example, the exterior angle at in the following diagram.

We know that the sum of the measures of the interior angles in a triangle is . So,

Similarly, we know that the exterior and interior angles at make up a straight angle, so the sum of their measures is . Thus,

Since the left-hand sides of both equations are equal to , we can equate the left-hand sides of the equations to get

We can subtract from both sides of the equation to get

We now note that , , since they are measures of angles that are not the zero angle. Since and are positive, we can subtract either positive number from the left-hand side of the equation to make it smaller. Hence,

We have proven that the measure of an exterior angle in any triangle is greater than the measure of either of the two nonadjacent interior angles of the triangle. We state this result formally as follows.

### Property: Comparing the Measures of the Interior and Exterior Angles in a Triangle

The measure of any exterior angle in a triangle is greater than the measure of either nonadjacent interior angle in the triangle.

In our next example, we will use this property to compare all of the angles in a diagram with measures smaller than a given angle.

### Example 2: Identifying Angles in a Triangle Less Than Another Angle

In the given figure, which angles must have smaller measures than ?

In this question, we need to identify all of the angles whose measures are less than the measure of . However, we are not given the measures of any of these angles, so instead, we will need to use the diagram and the relationships between the angles to compare their measures.

In particular, we recall that the measure of any exterior angle in a triangle is greater than the measure of either nonadjacent interior angle in the triangle.

We see that is an exterior angle in the following triangle.

Hence, its measure is greater than those of the two nonadjacent interior angles in this triangle, which are and .

We can show that none of the other angles have to have angle measures smaller than by considering an example. Let the triangle in red be an isosceles triangle and the measure of angle 1 be , so that we have the following:

We can add any line we want to construct the 4 missing angles. Since we have an isosceles triangle, we recall that the angle bisector and perpendicular bisector of the base are the same line. Adding this line in gives us the following:

We see that , which is larger than the measure of . So, these do not have smaller measures than . Next, , and we see that , , and combine to make a straight angle. Thus, their measures add to :

Substituting and yields

Subtracting from both sides of the equation and simplifying then gives

Therefore,

This is equal in measure to . So, these do not have smaller measures than .

Whilst the red triangle is not necessarily isosceles, we have demonstrated that there are circumstances in which the remaining angles may not have a measure smaller than .

Hence, only and must have measures smaller than .

In our next example, we will order the measures of the interior angles in a triangle using the measures of an interior and exterior angle.

### Example 3: Using Inequalities to Order the Angles in a Triangle

Which of the following inequalities is correct?

We can answer this question by determining the measures of each of the angles in the triangle.

First, we recall that the sum of the measures of angles that make up a straight angle is . So,

Subtracting from both sides of the equation yields

We then recall that the sum of the measures of the interior angles in a triangle is . Thus,

Therefore, we can see that and . We can combine these into one inequality written as , which is option B.

In our next example, we will use the properties of isosceles triangles to compare the measures of two angles in a given diagram.

### Example 4: Proving an Inequality in a Triangle

Use <, =, or > to complete the statement: If and , then .

We start by noticing that , so triangle is an isosceles triangle. We then recall that the angles opposite the congruent sides in an isosceles triangle are congruent, so . We can add this congruence and the given angle measures to the diagram as follows:

We can determine the measure of the angle at by recalling that the sum of the measures of the interior angles in a triangle is . So,

We can then add this onto the diagram as shown.

If we call the measure of the congruent angles , then by comparing the angles at and with the smaller angles at these vertices we can see that

Adding to both sides of the second equation gives

Now, the left-hand sides of the two equations are equal to , so they must be equal. Hence,

Subtracting from both sides of the equation then yields

Thus, is smaller than , so

In our final example, we will use the properties of transversals of parallel lines to compare the measures of two angles in a given diagram.

### Example 5: Proving an Inequality in a Triangle

Use <, =, or > to fill in the blank: .

Let’s first mark the two angles we want to compare on the diagram.

It appears the angle at is larger than the angle at ; however, we want to prove this result. We can do this by noting that we are given a pair of parallel lines and is a transversal of these parallel lines. We then recall that alternate interior angles in a transversal of parallel lines are congruent. So, .

We know that . Substituting gives

Finally, since , we can say that

Let’s finish by recapping some of the important points from this explainer.

### Key Points

• The axioms of inequalities tell us for any real numbers , , , and , we have the following:
• We can compare the sizes of any two real numbers. In other words, either , , or .
• Inequalities and equalities are transitive relations on the set of real numbers. This means that if and , then we must have that . This is called the transitivity axiom.
• Adding a constant to or subtracting a constant from both sides of an inequality gives an equivalent inequality. For example, if , then and .
• Multiplying or dividing both sides of an inequality by a positive constant yields an equivalent inequality. For example, if and , then and .
• We can combine the ideas of the transitive axiom with the other axioms. For example, if and , then .
• The measure of any exterior angle in a triangle is greater than the measure of either nonadjacent interior angle in the triangle.