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# Angle Sum Property of a Triangle

• Last Updated : 17 Jan, 2023

Angle Sum Property of a Triangle is the special property of a triangle that is used to find the value of an unknown angle in the triangle. The property states that “The Sum of All the Angles of a Triangle is equal to 180Âº.”

A triangle is a simple polygon with three sides and three angles. Angle Sum Property of a Triangle is the most widely used property of a triangle. Let’s learn about the triangle sum property in detail in this article.

## What is the Angle Sum Property?

For a closed polygon, the sum of all the interior angles is dependent on the sides of the polygon. In a triangle sum of all the interior angles is equal to 180 degrees. This property holds true for all types of triangles such as acute-angled triangles, right-angled triangles, scalene triangles, and others. This property is very useful in finding the unknown angle of the triangle if two angles of the triangle are given.

## Angle Sum Property Formula

The angle sum property formula used for any polygon is given by the expression,

Sum of Interior Angle = (n âˆ’ 2) Ã— 180Â°

where ‘n’ is the number of sides of the polygon.

According to this property, the sum of the interior angles of the polygon depends on how many triangles are formed inside the polygon, i.e. for 1 triangle the sum of interior angles is 1Ã—180Â° for two triangles inside the polygon the sum of interior angles is 2Ã—180Â° similarly for a polygon of ‘n’ sides, (n – 2) triangles are formed inside it.

Example: Find the sum of the interior angles for the pentagon.

Solution:

Pentagon has 5 sides.

So, n = 5

Thus, n – 2 = 5 – 2 = 3 triangles are formed.

Sum of Interior Angle = (n âˆ’ 2) Ã— 180Â°

Sum of Interior Angle = (5 âˆ’ 2) Ã— 180Â°

Sum of Interior Angle = 3 Ã— 180Â° = 540Â°

## Proof of Angle Sum Property

Theorem 1: The angle sum property of a triangle states that the sum of interior angles of a triangle is 180Â°.

Proof:

The sum of all the angles of a triangle is equal to 180Â° this theorem can be proved by the below-shown figure.

Follow the steps given below to prove the angle sum property in the triangle.

Step 1: Draw a line parallel to any given side of a triangle let’s make a line AB parallel to side RQ of the triangle.

Step 2: We know that sum of all the angles in a straight line is 180Â°. So, âˆ APR + âˆ RPQ + âˆ BPQ = 180Â°

Step 3: In the given figure as we can see that side AB is parallel to RQ and RP, and QP act as a transversal. So we can see that angle âˆ APR = âˆ PRQ and âˆ BPQ = âˆ PQR by the property of alternate interior angles we have studied above.

Therefore we can prove that

âˆ APR + âˆ RPQ + âˆ BPQ = 180Â° and Hence âˆ PRQ + âˆ RPQ + âˆ PQR = 180Â°

Example: In the given triangle PQR if the given is âˆ PQR = 30Â°, âˆ QRP = 70Â°then find the unknown âˆ RPQ

Solution:

As we know that, sum of all the angle of triangle is 180Â°

âˆ PQR + âˆ QRP + âˆ RPQ = 180Â°
30Â° + 70Â° + âˆ RPQ = 180Â°
100Â° + âˆ RPQ = 180Â°
âˆ RPQ = 180Â° – 100Â°
âˆ RPQ = 80Â°

Theorem 2: If any side of a triangle is extended, then the exterior angle so formed is the sum of the two opposite interior angles of the triangle.

Proof:

As we have proved the sum of all the interior angles of a triangle is 180Â° (âˆ ACB + âˆ ABC + âˆ BAC = 180Â°) and we can also see in figure 7 that âˆ ACB + âˆ ACD = 180Â° due to the straight line. By the above two equations, we can conclude that

âˆ ACD = 180Â° – âˆ ACB

âˆ ACD = 180Â° – (180Â° – âˆ ABC – âˆ CAB)

âˆ ACD = âˆ ABC + âˆ CAB

Hence proved that If any side of a triangle is extended, then the exterior angle so formed is the sum of the two opposite interior angles of the triangle.

Example: In the triangle, ABC âˆ BAC = 60Â° and âˆ ABC = 70Â° then find the measure of angle âˆ ACB.

Solution:

The solution to this problem can be approached in two ways:

Method 1: By angle sum property of a triangle we know âˆ ACB + âˆ ABC + âˆ BAC = 180Â°

So therefore âˆ ACB = 180Â° – âˆ ABC – âˆ BAC

âˆ ACB = 180Â° – 70Â° – 60Â°

âˆ ACB = 50Â°

Method 2: By exterior angle sum property of a triangle, we know that âˆ ACD = âˆ BAC + âˆ ABC

âˆ ACD = 70Â° + 60Â°

âˆ ACD = 130Â°

âˆ ACB = 180Â° – âˆ ACD

âˆ ACB = 180Â° – 130Â°

âˆ ACB = 50Â°

Also, Check

## Solved Example on Angle Sum Property of a Triangle

Example 1: It is given that a transversal line cuts a pair of parallel lines and the âˆ 1: âˆ 2 = 4: 5 as shown in figure 9. Find the measure of the âˆ 3?

Solution:

As we are given that the given pair of a line are parallel so we can see that âˆ 1 and âˆ 2 are consecutive interior angles and we have already studied that consecutive interior angles are supplementary.

Therefore let us assume the measure of âˆ 1 as ‘4x’ therefore âˆ 2 would be ‘5x’

Given,  âˆ 1 : âˆ 2 = 4 : 5.

âˆ 1 + âˆ 2 = 180Â°

4x + 5x = 180Â°

9x = 180Â°

x = 20Â°

Therefore âˆ 1 = 4x = 4 Ã— 20Â° = 80Â° and âˆ 2 = 5x = 5 Ã— 20Â° = 100Â°.

As we can clearly see in the figure that âˆ 3  and âˆ 2 are alternate interior angles so âˆ 3 = âˆ 2

âˆ 3 = 100Â°.

Example 2: As shown in figure 10  angle APQ=120Â° and angle QRB=110Â°. Find the measure of the angle PQR given that the line AP is parallel to line RB.

Solution:

As we are given that line AP is parallel to line RB

We know that the line perpendicular to one would surely be perpendicular to the other. So let us make a line perpendicular to both the parallel line as shown in the picture.

Now as we can clearly see that

âˆ APM + âˆ MPQ = 120Â° and as PM is perpendicular to line AP so âˆ APM = 90Â° therefore,

âˆ MPQ = 120Â° – 90Â° = 30Â°.

Similarly, we can see that âˆ ORB = 90Â° as OR is perpendicular to line RB therefore,

âˆ QRO = 110Â° – 90Â° = 20Â°.

Line OR is parallel to line QN and MP therefore,

âˆ PQN = âˆ MPQ as they both are alternate interior angles. Similarly,

âˆ NQR = âˆ ORQ

Thus, âˆ PQR = âˆ PQN + âˆ NQR

âˆ PQR = 30Â° + 20Â°

âˆ PQR = 50Â°

## FAQs on Angle Sum Property

### Question 1: What is the Angle Sum Property of a Triangle?

Angle Sum Property of a triangle states that the sum of all the interior angles of a triangle is equal to 180Â°.  For example, In a triangle PQR, âˆ P + âˆ Q + âˆ R = 180Â°.

### Question 2: What is the Angle Sum Property of a Polygon?

The angle sum property of a Polygon states that for any polygon with side ‘n’ the sum of all its interior angle is given by,

Sum of all the interior angles of a polygon (side n) = (n-2) Ã— 180Â°

### Question 3: What is the use of the angle sum property?

The angle sum property of a triangle is used to find the unknown angle of a triangle when two angles are given.

### Question 4: Who discovered the angle sum property of a triangle?

The proof for triangle sum property was first published by, Bernhard Friedrich Thibaut in the second edition of his Grundriss der Reinen Mathematik