What is the exterior angle theorem?
Editor’s Note: “Exterior angle theorem” was published on [date]. The exterior angle theorem states that the measure of an exterior angle of a triangle is equal to the sum of the measures of the two remote interior angles.
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Key takeaways
Exterior Angle Theorem  

Definition  The measure of an exterior angle of a triangle is equal to the sum of the measures of the two remote interior angles. 
Formula  x = A + B 
Example 
In the triangle below, x is an exterior angle. A and B are the two remote interior angles. x = A + B 
Main article topics
 The proof of the exterior angle theorem
 Applications of the exterior angle theorem
 Common misconceptions about the exterior angle theorem
Exterior Angle Theorem
The exterior angle theorem is a fundamental theorem in geometry that relates the measures of the exterior and interior angles of a triangle. It is a powerful tool that can be used to solve a variety of problems. Here are 10 key aspects of the exterior angle theorem:
 Definition: The measure of an exterior angle of a triangle is equal to the sum of the measures of the two remote interior angles.
 Formula: x = A + B, where x is the exterior angle and A and B are the two remote interior angles.
 Proof: The proof of the exterior angle theorem is based on the fact that the sum of the angles in a triangle is 180 degrees.
 Converse: The converse of the exterior angle theorem is also true: If the measure of an exterior angle of a triangle is equal to the sum of the measures of the two remote interior angles, then the triangle is isosceles.
 Applications: The exterior angle theorem can be used to solve a variety of problems, such as finding the measure of an unknown angle in a triangle, determining whether a triangle is isosceles, and finding the area of a triangle.
 Limitations: The exterior angle theorem only applies to triangles. It does not apply to other polygons.
 History: The exterior angle theorem was first discovered by the Greek mathematician Euclid around 300 BC.
 Importance: The exterior angle theorem is an important theorem in geometry. It is used in a variety of applications, and it is essential for understanding the properties of triangles.

Example: In the triangle below, x is an exterior angle. A and B are the two remote interior angles.
x = A + B  Connection to other theorems: The exterior angle theorem is related to a number of other theorems in geometry, such as the triangle inequality theorem and the Pythagorean theorem.
The exterior angle theorem is a versatile and powerful theorem that can be used to solve a variety of problems. It is an essential tool for anyone who wants to understand the properties of triangles.
Definition
This definition is the foundation of the exterior angle theorem. It tells us that the measure of an exterior angle of a triangle is determined by the measures of the two interior angles that are not adjacent to it. This is an important relationship because it allows us to find the measure of an unknown exterior angle if we know the measures of the two remote interior angles.
For example, let’s say we have a triangle with interior angles measuring 60 degrees and 70 degrees. We can use the exterior angle theorem to find the measure of the exterior angle that is opposite the 60degree interior angle. According to the theorem, the measure of this exterior angle is equal to the sum of the measures of the two remote interior angles, which is 60 degrees + 70 degrees = 130 degrees.
The exterior angle theorem is a versatile tool that can be used to solve a variety of problems. It is an essential tool for anyone who wants to understand the properties of triangles.
Key insights:
 The exterior angle theorem is a fundamental theorem in geometry.
 It is used to find the measure of an unknown exterior angle if we know the measures of the two remote interior angles.
 The exterior angle theorem is a powerful tool that can be used to solve a variety of problems.
Exterior Angle Theorem  

Definition  The measure of an exterior angle of a triangle is equal to the sum of the measures of the two remote interior angles. 
Formula  x = A + B, where x is the exterior angle and A and B are the two remote interior angles. 
Example 
In the triangle below, x is an exterior angle. A and B are the two remote interior angles. x = A + B 
Formula
The formula x = A + B is a concise mathematical expression of the exterior angle theorem. It states that the measure of an exterior angle of a triangle is equal to the sum of the measures of the two remote interior angles. This formula is a powerful tool that can be used to solve a variety of problems involving triangles.
 Calculating Exterior Angles: The formula can be used to calculate the measure of an exterior angle if the measures of the two remote interior angles are known. This is useful in situations where one or more of the exterior angles is unknown.
 Determining Triangle Properties: The formula can be used to determine whether a triangle is isosceles, scalene, or equilateral. For example, if the measure of an exterior angle is 180 degrees, then the triangle is isosceles.
 Solving Geometric Problems: The formula can be used to solve a variety of geometric problems, such as finding the area of a triangle or the length of a side.
The formula x = A + B is a fundamental tool in geometry. It is used in a wide variety of applications, and it is essential for understanding the properties of triangles.
Proof
The proof of the exterior angle theorem is based on the fact that the sum of the angles in a triangle is 180 degrees. This is a fundamental property of triangles that can be proven using a variety of methods. One common method is to use the fact that the angles in a straight line add up to 180 degrees.Consider a triangle ABC. Let BAC be the exterior angle at vertex A. We can extend the side AB to form a straight line. This creates two new angles: BAD and DAC.Since the angles in a straight line add up to 180 degrees, we have:BAD + DAC = 180 degreesBut BAD is equal to BAC, since they are vertical angles. Therefore, we have:BAC + DAC = 180 degreesBAC is the exterior angle at vertex A, and DAC is the remote interior angle at vertex A. Therefore, we have proven that the measure of an exterior angle of a triangle is equal to the sum of the measures of the two remote interior angles.This proof is based on the fundamental property of triangles that the sum of the angles is 180 degrees. This property is essential for understanding the geometry of triangles and for proving many other theorems about triangles.
Converse
The converse of the exterior angle theorem is an important property of triangles that is closely related to the exterior angle theorem itself. The exterior angle theorem states that the measure of an exterior angle of a triangle is equal to the sum of the measures of the two remote interior angles. The converse of this theorem states that if the measure of an exterior angle of a triangle is equal to the sum of the measures of the two remote interior angles, then the triangle is isosceles.
This property is important because it allows us to determine whether a triangle is isosceles by measuring the exterior angle and the two remote interior angles. If the exterior angle is equal to the sum of the two remote interior angles, then the triangle is isosceles. This can be useful in situations where it is difficult or impossible to measure the sides of the triangle directly.
For example, consider a triangle with an exterior angle that measures 120 degrees. If we measure the two remote interior angles and find that they both measure 60 degrees, then we know that the triangle is isosceles. This is because the exterior angle is equal to the sum of the two remote interior angles, which is 60 degrees + 60 degrees = 120 degrees.
The converse of the exterior angle theorem is a powerful tool that can be used to determine whether a triangle is isosceles. It is a useful property that is closely related to the exterior angle theorem itself.
Exterior Angle Theorem  Converse of Exterior Angle Theorem  

Definition  The measure of an exterior angle of a triangle is equal to the sum of the measures of the two remote interior angles.  If the measure of an exterior angle of a triangle is equal to the sum of the measures of the two remote interior angles, then the triangle is isosceles. 
Formula  x = A + B, where x is the exterior angle and A and B are the two remote interior angles.  x = A + B, where x is the exterior angle and A and B are the two remote interior angles. 
Example 
In the triangle below, x is an exterior angle. A and B are the two remote interior angles. x = A + B 
In the triangle below, x is an exterior angle. A and B are the two remote interior angles. If x = A + B, then the triangle is isosceles. 
Applications
The exterior angle theorem is a fundamental theorem in geometry that relates the measures of the exterior and interior angles of a triangle. It is a powerful tool that can be used to solve a variety of problems, including:

Finding the measure of an unknown angle in a triangle
The exterior angle theorem can be used to find the measure of an unknown angle in a triangle if the measures of the other two angles are known. This is useful in situations where it is difficult or impossible to measure the unknown angle directly. 
Determining whether a triangle is isosceles
The exterior angle theorem can be used to determine whether a triangle is isosceles. A triangle is isosceles if two of its sides are equal. If the measure of an exterior angle of a triangle is equal to the sum of the measures of the two remote interior angles, then the triangle is isosceles. 
Finding the area of a triangle
The exterior angle theorem can be used to find the area of a triangle. The area of a triangle is given by the formula A = 1/2 bh, where b is the length of the base and h is the height of the triangle. The height of a triangle can be found using the exterior angle theorem. If an exterior angle of a triangle is 90 degrees, then the height of the triangle is equal to the length of the side opposite the exterior angle.
These are just a few of the many applications of the exterior angle theorem. It is a versatile and powerful theorem that can be used to solve a wide variety of problems in geometry.
Limitations
The exterior angle theorem is a fundamental theorem in geometry that relates the measures of the exterior and interior angles of a triangle. It is a powerful tool that can be used to solve a variety of problems involving triangles. However, it is important to note that the exterior angle theorem only applies to triangles. It does not apply to other polygons.

Reason for limitation
The exterior angle theorem relies on the fact that the sum of the interior angles of a triangle is 180 degrees. This property is unique to triangles. Other polygons have different properties that affect the relationship between the exterior and interior angles.

Implications of limitation
The limitation of the exterior angle theorem to triangles means that it cannot be used to solve problems involving other polygons. For example, it cannot be used to find the measure of an unknown angle in a quadrilateral or to determine whether a quadrilateral is a parallelogram.

Alternative theorems for other polygons
While the exterior angle theorem does not apply to other polygons, there are other theorems that can be used to solve problems involving the angles of polygons. For example, the exterior angle sum theorem states that the sum of the exterior angles of a convex polygon is 360 degrees.
The limitation of the exterior angle theorem to triangles is an important consideration when using this theorem to solve problems. It is important to understand the limitations of any theorem in order to use it effectively.
History
The exterior angle theorem is a fundamental theorem in geometry that relates the measures of the exterior and interior angles of a triangle. It is a powerful tool that can be used to solve a variety of problems, such as finding the measure of an unknown angle in a triangle, determining whether a triangle is isosceles, and finding the area of a triangle.
The exterior angle theorem was first discovered by the Greek mathematician Euclid around 300 BC. Euclid’s discovery of the exterior angle theorem was a major breakthrough in geometry. It allowed mathematicians to develop a more complete understanding of the properties of triangles and to solve a wider range of problems.
The exterior angle theorem is an important component of geometry. It is used in a wide variety of applications, such as architecture, engineering, and surveying. It is also used in many different branches of mathematics, such as trigonometry and calculus.
Understanding the history of the exterior angle theorem can help us to appreciate the development of geometry as a subject. It can also help us to understand the importance of the exterior angle theorem in mathematics and its applications.
Here is a table that summarizes the key insights about the exterior angle theorem:
Key Insight  Explanation 

The exterior angle theorem was first discovered by Euclid around 300 BC.  This discovery was a major breakthrough in geometry. 
The exterior angle theorem is a fundamental theorem in geometry.  It is used to solve a variety of problems involving triangles. 
The exterior angle theorem is used in a wide range of applications.  It is used in architecture, engineering, surveying, and many different branches of mathematics. 
Importance
The exterior angle theorem is an important theorem in geometry because it provides a relationship between the measures of the exterior and interior angles of a triangle. This relationship can be used to solve a variety of problems involving triangles, such as finding the measure of an unknown angle, determining whether a triangle is isosceles, and finding the area of a triangle.
The exterior angle theorem is used in a wide variety of applications, including architecture, engineering, and surveying. In architecture, the exterior angle theorem is used to design roofs and other structures. In engineering, the exterior angle theorem is used to design bridges and other structures. In surveying, the exterior angle theorem is used to measure land and create maps.
Understanding the exterior angle theorem is essential for understanding the properties of triangles. The exterior angle theorem can be used to prove other theorems about triangles, such as the triangle inequality theorem and the Pythagorean theorem. The exterior angle theorem can also be used to solve a variety of problems involving triangles.
Here are some examples of how the exterior angle theorem can be used to solve problems:
 To find the measure of an unknown angle in a triangle, you can use the exterior angle theorem to add the measures of the two remote interior angles.
 To determine whether a triangle is isosceles, you can use the exterior angle theorem to check if the measure of an exterior angle is equal to the sum of the measures of the two remote interior angles.
 To find the area of a triangle, you can use the exterior angle theorem to find the height of the triangle and then use the formula for the area of a triangle.
The exterior angle theorem is a versatile and powerful theorem that can be used to solve a variety of problems involving triangles. It is an essential tool for anyone who wants to understand the properties of triangles.
Key insights:
 The exterior angle theorem is an important theorem in geometry.
 The exterior angle theorem is used in a variety of applications.
 The exterior angle theorem is essential for understanding the properties of triangles.
Exterior Angle Theorem  

Definition  The measure of an exterior angle of a triangle is equal to the sum of the measures of the two remote interior angles. 
Formula  x = A + B, where x is the exterior angle and A and B are the two remote interior angles. 
Example 
In the triangle below, x is an exterior angle. A and B are the two remote interior angles. x = A + B 
Example
The exterior angle theorem states that the measure of an exterior angle of a triangle is equal to the sum of the measures of the two remote interior angles. In other words, the exterior angle is equal to the sum of the two angles that are not adjacent to it.
The example given is a simple illustration of the exterior angle theorem. In the triangle below, x is the exterior angle. A and B are the two remote interior angles. The exterior angle theorem tells us that the measure of angle x is equal to the sum of the measures of angles A and B.
In the example, we can see that angle x is 120 degrees. Angle A is 60 degrees and angle B is 60 degrees. This verifies the exterior angle theorem, as 120 degrees is equal to 60 degrees + 60 degrees.
The exterior angle theorem is a useful tool for solving problems involving triangles. It can be used to find the measure of an unknown angle, to determine whether a triangle is isosceles, and to find the area of a triangle.
Here are some examples of how the exterior angle theorem can be used to solve problems:
 To find the measure of an unknown angle in a triangle, you can use the exterior angle theorem to add the measures of the two remote interior angles.
 To determine whether a triangle is isosceles, you can use the exterior angle theorem to check if the measure of an exterior angle is equal to the sum of the measures of the two remote interior angles.
 To find the area of a triangle, you can use the exterior angle theorem to find the height of the triangle and then use the formula for the area of a triangle.
The exterior angle theorem is a versatile and powerful theorem that can be used to solve a variety of problems involving triangles. It is an essential tool for anyone who wants to understand the properties of triangles.
Key insights:
 The exterior angle theorem states that the measure of an exterior angle of a triangle is equal to the sum of the measures of the two remote interior angles.
 The exterior angle theorem can be used to solve a variety of problems involving triangles.
 The exterior angle theorem is an essential tool for anyone who wants to understand the properties of triangles.
Exterior Angle Theorem  

Definition  The measure of an exterior angle of a triangle is equal to the sum of the measures of the two remote interior angles. 
Formula  x = A + B, where x is the exterior angle and A and B are the two remote interior angles. 
Example 
In the triangle below, x is an exterior angle. A and B are the two remote interior angles. x = A + B 
Connection to other theorems
The exterior angle theorem is a fundamental theorem in geometry that relates the measures of the exterior and interior angles of a triangle. It is a powerful tool that can be used to solve a variety of problems involving triangles.

Triangle Inequality Theorem
The triangle inequality theorem states that the sum of the lengths of any two sides of a triangle is greater than the length of the third side. This theorem can be proven using the exterior angle theorem. 
Pythagorean Theorem
The Pythagorean theorem states that in a right triangle, the square of the length of the hypotenuse is equal to the sum of the squares of the lengths of the other two sides. This theorem can also be proven using the exterior angle theorem.
These are just two examples of the many connections between the exterior angle theorem and other theorems in geometry. These connections highlight the importance of the exterior angle theorem as a fundamental building block of geometry.
Exterior Angle Theorem FAQs
This section addresses frequently asked questions about the exterior angle theorem, a fundamental concept in geometry.
Question 1: What is the exterior angle theorem?
Answer: The exterior angle theorem states that the measure of an exterior angle of a triangle is equal to the sum of the measures of the two remote interior angles.
Question 2: What is the formula for the exterior angle theorem?
Answer: The formula for the exterior angle theorem is x = A + B, where x is the exterior angle and A and B are the two remote interior angles.
Question 3: How can I use the exterior angle theorem?
Answer: The exterior angle theorem can be used to solve a variety of problems involving triangles, such as finding the measure of an unknown angle, determining whether a triangle is isosceles, and finding the area of a triangle.
Question 4: What are some limitations of the exterior angle theorem?
Answer: The exterior angle theorem only applies to triangles. It does not apply to other polygons.
Question 5: Who discovered the exterior angle theorem?
Answer: The exterior angle theorem was first discovered by the Greek mathematician Euclid around 300 BC.
Question 6: Why is the exterior angle theorem important?
Answer: The exterior angle theorem is important because it is a fundamental theorem in geometry. It is used in a variety of applications, and it is essential for understanding the properties of triangles.
Summary: The exterior angle theorem is a versatile and powerful theorem that can be used to solve a variety of problems involving triangles. It is an essential tool for anyone who wants to understand the properties of triangles.
Transition to the next article section: The next section will explore the applications of the exterior angle theorem in more detail.
Tips on Using the Exterior Angle Theorem
The exterior angle theorem is a fundamental theorem in geometry that can be used to solve a variety of problems involving triangles. Here are a few tips on how to use the exterior angle theorem effectively:
Tip 1: Understand the definition of the exterior angle theorem
The exterior angle theorem states that the measure of an exterior angle of a triangle is equal to the sum of the measures of the two remote interior angles. This means that the exterior angle is equal to the sum of the two angles that are not adjacent to it.
Tip 2: Use the formula for the exterior angle theorem
The formula for the exterior angle theorem is x = A + B, where x is the exterior angle and A and B are the two remote interior angles. This formula can be used to find the measure of an unknown exterior angle if the measures of the two remote interior angles are known.
Tip 3: Apply the exterior angle theorem to solve problems
The exterior angle theorem can be used to solve a variety of problems involving triangles. For example, it can be used to find the measure of an unknown angle, to determine whether a triangle is isosceles, and to find the area of a triangle.
Tip 4: Be aware of the limitations of the exterior angle theorem
The exterior angle theorem only applies to triangles. It does not apply to other polygons.
Tip 5: Practice using the exterior angle theorem
The best way to learn how to use the exterior angle theorem is to practice using it. There are many online resources and worksheets that can help you practice.
By following these tips, you can learn how to use the exterior angle theorem to solve a variety of problems involving triangles.
Key takeaways:
 Understand the definition of the exterior angle theorem.
 Use the formula for the exterior angle theorem.
 Apply the exterior angle theorem to solve problems.
 Be aware of the limitations of the exterior angle theorem.
 Practice using the exterior angle theorem.
Conclusion:
The exterior angle theorem is a powerful tool that can be used to solve a variety of problems involving triangles. By understanding the definition, formula, and applications of the exterior angle theorem, you can use it to your advantage in your geometry studies.
Conclusion
The exterior angle theorem is a fundamental theorem in geometry that relates the measures of the exterior and interior angles of a triangle. It states that the measure of an exterior angle of a triangle is equal to the sum of the measures of the two remote interior angles. This theorem is a powerful tool that can be used to solve a variety of problems involving triangles.
In this article, we have explored the exterior angle theorem in detail. We have discussed its definition, formula, and applications. We have also provided tips on how to use the exterior angle theorem effectively. We encourage you to practice using this theorem so that you can become proficient in solving problems involving triangles.