Introduction to Exterior Angle Theorem on Vedantu
An angle is a geometrical figure with two rays and is generated from a common point. It is measured in degrees or radians and basically measures the degree of turn between the two sides of an angle. Now, the turns can be measured by finding out the interior angles or the exterior angle of straight lines. In this article, we shall be learning about exterior angles and the related concepts.
Table of Content
Introduction
What is Exterior Angle
Exterior Angle Property
Exterior Angle Property of a Triangle
Properties of Exterior Angle:
Exterior Angle Theorem
Exterior Angle Theorem Proof
Solved Examples
Frequently asked questions
What is an Exterior Angle?
A triangle has three vertices or points. By joining these points we get three sides. The degree of turn between the sides when measured from the inside of the triangle or any object is its interior angle. Whereas, the angles or the degree of turn between the sides, when measured on the outer angles of an object, is its exterior angle.
Exterior Angle Property
The exterior angle theorem is amongst the most basic theorems of triangles in geometry. Before we begin the discussion, let us have a look at what a triangle is. A polygon is called a plane figure that is bounded by the finite number of line segments for forming a closed figure. The smallest polygon is known as a triangle since there are three line segments that are bound to it. The triangle is the smallest polygon which is bounded by three different line segments. It consists of three edges and three vertices. The exterior angle of the triangle is formed between any of the sides of the triangle and the extension of the adjacent side. We will learn in this lesson about the exterior angle theorem, exterior angle property, exterior angle theorem proof, and look at the examples.
Exterior Angle Property of a Triangle
Let us first learn about the exterior angle property before we learn about the exterior angle theorem.
An exterior angle of a triangle is equal to the angle formed between one side of the triangle and the extension of the adjacent side. Consider the figure given below.
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Properties of Exterior Angle
The properties of the exterior angle is given as follows:
The exterior angle of a given triangle equals the sum of the opposite interior angles of that triangle.
If an equivalent angle is taken at each vertex of the triangle, the exterior angles add to 360° in all the cases. In fact, this statement is true for any given convex polygon and not just triangles.
Exterior Angle Theorem
Let us learn more about the exterior angles and the exterior angle theorem in detail.
An exterior angle is an angle that is formed between one side of the polygon and the extension of the adjacent side.
In all the known polygons, there are two different sets of exterior angles, one that goes around the clockwise direction and the other that goes around the counterclockwise direction.
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You can notice here that the interior angle and its adjacent exterior angle both tend to form a linear pair and their sum adds up to 180°.
m∠1 + m∠2 = 180∘
The exterior angle theorem states that the sum total of all the remote interior angles of the triangle is equal to the non-adjacent exterior angle of that triangle. From the figure above, it means that m∠A + m∠B = m∠ACD. Given below is the proof of the exterior angle theorem. From the theorem’s proof, you would see that this theorem is the combination of both the Triangle Sum Theorem and the Linear Pair Postulate.
Exterior Angle Theorem Proof
Let us look at the exterior angle proof.
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Given is the △ABC with the exterior angle ∠ACD
We have to prove that m∠A + m∠B = m∠ACD
Given below is the proof:
Hence, it is proved that m∠A + m∠B = m∠ACD
Solved Examples
Take a look at the solved examples given below to understand the concept of the exterior angles and the exterior angle theorem.
Example 1
Find the measure of the unknown numbered interior and exterior angles in the given triangle below.
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Solution:
m∠1 + 92∘ = 180∘ through the Linear Pair Postulate
Hence, m∠1 = 88∘
m∠2 + 123∘ = 180∘ through the Linear Pair Postulate
Hence, m∠2 = 57∘
m∠1 + m∠2 + m∠3 =180∘through the Triangle Sum Theorem
Hence, 88∘ + 57∘ + m∠3 = 180∘ and also m∠3 = 35∘
m∠3 + m∠4 = 180∘ through the Linear Pair Postulate
Hence, m∠4 = 145∘
Example 2
Determine the value of p in the triangle below
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Solution:
First, you need to find the missing exterior angle and you can call it x. Then set up an equation with the help of the Exterior Angle Sum Theorem
130∘ + 110∘ + x = 360∘
= x = 360∘ − 130∘ − 110∘
Hence, x = 120∘
x and p are the supplementary angles and add up to 180∘
x + p = 180∘
= 120∘ + p = 180∘
Hence, p = 60∘
Example 3
Determine m∠C
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Solution:
By using the exterior angle theorem, you get m∠C + 16∘ = 121∘
By subtracting 16∘ from both the sides, you get m∠C = 105∘
FAQs on Exterior Angle Theorem
1. What is the relation between the interior and the exterior angle of a triangle?
Let us try to understand it with an example. Take any triangle with sides ABC. The sum of the interior triangle will be 180 degrees. And the sum of one side of an exterior triangle is equal to the sum of the opposite interior angles of the triangle. Try drawing the triangle and you will be able to understand the concept better.
2. What are the limitations of an exterior angle theorem?
Every property or a theorem comes with its own set of limitations. The limitations of an exterior angle theorem are that it is invalid for any spherical geometrical shape or an elliptical shape. However, apart from this, the theorem is applicable in all the other geometrical shapes.
3. What is an exterior angle inequality theorem?
We all know that when an exterior angle is equal to the sum of the two interior opposite angles, it is the exterior angle theorem. Bur, when the angle of the exterior angle is more than any or both of the opposite angles of a triangle. It is referred to as the exterior angle inequality theorem.
4. What is the sum of all the exterior angles of a convex polygon?
If we see the interior angles of any polygon, the sum of the angles may vary for different polygons. But to your surprise when we see the exterior angle of any polygon, the sum is always 360 degrees. The reason is that the complete turn of any polygon irrespective of its shape is 360 degrees.
5. Can we find the sides of a regular polygon with the measure of an exterior angle?
The process to find the sides of any polygon is very simple when you have the measure of the angle. All you have to do is divide 360 by the measure of the exterior angle and the resultant number will be the number of sides of the polygon. For example, if the measure of the angle is 60, divide 360 by 60. You will get 6 as the answer which is the number of the sides of a hexagon.
6. What is the Statement of the Exterior Angle Theorem?
The exterior angle of a triangle is formed between any side of that given triangle and the extension of the triangle’s adjacent side.
The exterior angle theorem states that the exterior angle of the given triangle equals to the sum of the two opposite interior angles present in that triangle.
The following diagram shows the exterior angle theorem.
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7. What is the Use of the Exterior Angle Theorem?
The exterior angle of a given triangle is formed when a side is extended outwards. The exterior angle theorem states that the exterior angle of the given triangle is equal to the sum total of its opposite interior angles.
You can derive the exterior angle theorem with the help of the information that
The angles on the straight line add up to 180°
The interior angles of the given triangle add up to 180°
8. What is the Sum of All the Exterior Angles of a Triangle?
The exterior angle of a given polygon is the angle outside the polygon which is formed by one of its sides and the extension of its adjacent side. The sum of all the exterior angles of the given polygon results to 360° in all the cases irrespective of the number of sides of the polygon. Hence, the answer is 360°.