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Isosceles Triangles

Properties of triangles with two equal sides.

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Isosceles Triangles

Isosceles Triangles 

An isosceles triangle is a triangle that has at least two congruent sides. The congruent sides of the isosceles triangle are called the legs. The other side is called the base and the angles between the base and the congruent sides are called base angles. The angle made by the two legs of the isosceles triangle is called the vertex angle.

Investigation: Isosceles Triangle Construction

Tools Needed: pencil, paper, compass, ruler, protractor

  1. Using your compass and ruler, draw an isosceles triangle with sides of 3 in, 5 in and 5 in. Draw the 3 in side (the base) horizontally 6 inches from the top of the page.
  2. Now that you have an isosceles triangle, use your protractor to measure the base angles and the vertex angle.

The base angles should each be and the vertex angle should be .

Watch the first part of this video.

We can generalize this investigation into the Base Angles Theorem.

Base Angles Theorem: The base angles of an isosceles triangle are congruent.

To prove the Base Angles Theorem, we will construct the angle bisector through the vertex angle of an isosceles triangle.

Given: Isosceles triangle with

Prove:

Statement Reason
1. Isosceles triangle with Given

2. Construct angle bisector for

Every angle has one angle bisector
3. Definition of an angle bisector
4. Reflexive PoC
5. SAS
6. CPCTC

By constructing the angle bisector, , we designed two congruent triangles and then used CPCTC to show that the base angles are congruent. Now that we have proven the Base Angles Theorem, you do not have to construct the angle bisector every time. It can now be assumed that base angles of any isosceles triangle are always equal. Let’s further analyze the picture from step 2 of our proof.

Because , we know that by CPCTC. Thes two angles are also a linear pair, so they are congruent supplements, or each. Therefore, . Additionally, by CPCTC, so is the midpoint of . This means that is the perpendicular bisector of , in addition to being the angle bisector of .

Isosceles Triangle Theorem: The angle bisector of the vertex angle in an isosceles triangle is also the perpendicular bisector to the base.

The converses of the Base Angles Theorem and the Isosceles Triangle Theorem are both true.

Base Angles Theorem Converse: If two angles in a triangle are congruent, then the opposite sides are also congruent.

So, for a triangle , if , then . would be the vertex angle.

Isosceles Triangle Theorem Converse: The perpendicular bisector of the base of an isosceles triangle is also the angle bisector of the vertex angle.

In other words, if is isosceles, and , then .

Recognizing Congruent Angles 

Which two angles are congruent?

This is an isosceles triangle. The congruent angles, are opposite the congruent sides.

From the arrows we see that .

Measuring Vertex Angles 

If an isosceles triangle has base angles with measures of , what is the measure of the vertex angle?

Draw a picture and set up an equation to solve for the vertex angle, .

Measuring Base Angles 

If an isosceles triangle has a vertex angle with a measure of , what is the measure of each base angle?

Draw a picture and set up and equation to solve for the base angles, . Recall that the base angles are equal.

Examples

Example 1

Find the value of and the measure of each angle.

Set the angles equal to each other and solve for .

If , then the base angles are , or . The vertex angle is .

Example 2

Find the measure of .

The two sides are equal, so set them equal to each other and solve for .

 

Example 3

True or false: Base angles of an isosceles triangle can be right angles.

This statement is false. Because the base angles of an isosceles triangle are congruent, if one base angle is a right angle then both base angles must be right angles. It is impossible to have a triangle with two right () angles. The Triangle Sum Theorem states that the sum of the three angles in a triangle is . If two of the angles in a triangle are right angles, then the third angle must be and the shape is no longer a triangle.

Review 

Find the measures of and/or .

Determine if the following statements are true or false.

  1. Base angles of an isosceles triangle are congruent.
  2. Base angles of an isosceles triangle are complementary.
  3. Base angles of an isosceles triangle can be equal to the vertex angle.
  4. Base angles of an isosceles triangle are acute.

Complete the proofs below.

  1. Given: Isosceles , with base angles and is the angle bisector of Prove: is the perpendicular bisector of
  1. Given: Isosceles with and is the perpendicular bisector of Prove: is the angle bisector of

On the plane, plot the coordinates and determine if the given three points make a scalene or isosceles triangle.

  1. (-2, 1), (1, -2), (-5, -2)
  2. (-2, 5), (2, 4), (0, -1)
  3. (6, 9), (12, 3), (3, -6)
  4. (-10, -5), (-8, 5), (2, 3)
  5. (-1, 2), (7, 2), (3, 9)

Review (Answers)

To view the Review answers, open this PDF file and look for section 4.10. 

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Vocabulary

TermDefinition
Base Angles The base angles of an isosceles triangle are the angles formed by the base and one leg of the triangle.
Base Angles Theorem Converse The base angles theorem converse states if two angles in a triangle are congruent, then the sides opposite those angles are also congruent.
Isosceles Triangle Theorem The Isosceles Triangle Theorem states that the perpendicular bisector of the base of an isosceles triangle is also the angle bisector of the vertex angle.
Triangle Sum Theorem The Triangle Sum Theorem states that the three interior angles of any triangle add up to 180 degrees.
Vertical Angles Vertical angles are a pair of opposite angles created by intersecting lines.

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