Exterior angles of a triangle - Examples, Exercises and Solutions

Question Types:
Parts of a Triangle: Is it possible...?Parts of a Triangle: Using the median to calculate the perimeterParts of a Triangle: Using the theorem: The median of a triangle divides it into two triangles of the same areaSum and Difference of Angles: Is it possible...?Sum and Difference of Angles: Isosceles triangleParts of a Triangle: Median in an isosceles triangle - propertiesParts of a Triangle: Using parallel linesParts of a Triangle: Classify a triangle by its sidesParts of a Triangle: Determine the size of the given anglesParts of a Triangle: Identify the greater valueParts of a Triangle: Median in a right-angled triangleParts of a Triangle: Using properties of the medianSum and Difference of Angles: Using variablesParts of a Triangle: Identifying the sides in a triangle using median and heightSum and Difference of Angles: Calculate Angles in QuadrilateralsParts of a Triangle: Calculate Angles in QuadrilateralsParts of a Triangle: True / falseSum and Difference of Angles: Identifying and defining elementsSum and Difference of Angles: Using quadrilateralsSum and Difference of Angles: Angle bisectorParts of a Triangle: Identifying sides in a triangleSum and Difference of Angles: Applying the formulaSum and Difference of Angles: Generate a random angleParts of a Triangle: Using angles in a triangleSum and Difference of Angles: Identify the greater valueSum and Difference of Angles: Using angles in a triangleParts of a Triangle: Finding the size of angles in a triangleParts of a Triangle: Identifying and defining elementsSum and Difference of Angles: Finding the size of angles in a triangle

Exterior angles of a triangle

The exterior angle of a triangle is the one that is found between the original side and the extension of the side.

Key Properties:

  • The exterior angle is equal to the sum of the two interior angles of the triangle that are not adjacent to it.
  • Each exterior angle is supplementary to its adjacent interior angle, meaning their sum is 180180^\circ.
  • The sum of all exterior angles of a triangle is always 360360^\circ, no matter the shape of the triangle.

It is defined as follows:

α=A+Bα=∢A+∢B

A1 - Exterior angle of a triangle

Practice Exterior angles of a triangle

Examples with solutions for Exterior angles of a triangle

Exercise #1

Angle A is equal to 30°.
Angle B is equal to 60°.
Angle C is equal to 90°.

Can these angles form a triangle?

Video Solution

Step-by-Step Solution

We must first add the three angles to see if they equal 180 degrees:

30+60+90=180 30+60+90=180

The sum of the angles equals 180, therefore they can form a triangle.

Answer

Yes

Exercise #2

Angle A equals 56°.
Angle B equals 89°.
Angle C equals 17°.

Can these angles make a triangle?

Video Solution

Step-by-Step Solution

We add the three angles to see if they are equal to 180 degrees:

56+89+17=162 56+89+17=162

The sum of the given angles is not equal to 180, so they cannot form a triangle.

Answer

No.

Exercise #3

Angle A equals 90°.
Angle B equals 115°.
Angle C equals 35°.

Can these angles form a triangle?

Video Solution

Step-by-Step Solution

We add the three angles to see if they are equal to 180 degrees:

90+115+35=240 90+115+35=240
The sum of the given angles is not equal to 180, so they cannot form a triangle.

Answer

No.

Exercise #4

ABC is an isosceles triangle.

AD is the median.

What is the size of angle ADC ∢\text{ADC} ?

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Video Solution

Step-by-Step Solution

In an isosceles triangle, the median to the base is also the height to the base.

That is, side AD forms a 90° angle with side BC.

That is, two right triangles are created.

Therefore, angle ADC is equal to 90 degrees.

Answer

90

Exercise #5

What type of angle is α \alpha ?

αα

Step-by-Step Solution

Remember that an acute angle is smaller than 90 degrees, an obtuse angle is larger than 90 degrees, and a straight angle equals 180 degrees.

Since the lines are perpendicular to each other, the marked angles are right angles each equal to 90 degrees.

Answer

Straight

Exercise #6

Given the following triangle:

Write down the height of the triangle ABC.

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Video Solution

Step-by-Step Solution

An altitude in a triangle is the segment that connects the vertex and the opposite side, in such a way that the segment forms a 90-degree angle with the side.

If we look at the image it is clear that the above theorem is true for the line AE. AE not only connects the A vertex with the opposite side. It also crosses BC forming a 90-degree angle. Undoubtedly making AE the altitude.

Answer

AE

Exercise #7

Which of the following is the height in triangle ABC?

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Video Solution

Step-by-Step Solution

Let's remember the definition of height of a triangle:

A height is a straight line that descends from the vertex of a triangle and forms a 90-degree angle with the opposite side.

The sides that form a 90-degree angle are sides AB and BC. Therefore, the height is AB.

Answer

AB

Exercise #8

In a right triangle, the sum of the two non-right angles is...?

Video Solution

Step-by-Step Solution

In a right-angled triangle, there is one angle that equals 90 degrees, and the other two angles sum up to 180 degrees (sum of angles in a triangle)

Therefore, the sum of the two non-right angles is 90 degrees

90+90=180 90+90=180

Answer

90 degrees

Exercise #9

Can a triangle have two right angles?

Video Solution

Step-by-Step Solution

The sum of angles in a triangle is 180 degrees. Since two angles of 90 degrees equal 180, a triangle can never have two right angles.

Answer

No

Exercise #10

Look at the two triangles below. Is EC a side of one of the triangles?

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Video Solution

Step-by-Step Solution

Every triangle has 3 sides. First let's go over the triangle on the left side:

Its sides are: AB, BC, and CA.

This means that in this triangle, side EC does not exist.

Let's then look at the triangle on the right side:

Its sides are: ED, EF, and FD.

This means that in this triangle, side EC also does not exist.

Therefore, EC is not a side in either of the triangles.

Answer

No

Exercise #11

Fill in the blanks:

In an isosceles triangle, the angle between two ___ is called the "___ angle".

Step-by-Step Solution

In order to solve this problem, we need to understand the basic properties of an isosceles triangle.

An isosceles triangle has two sides that are equal in length, often referred to as the "legs" of the triangle. The angle formed between these two equal sides, which are sometimes referred to as the "sides", is called the "vertex angle" or sometimes more colloquially as the "main angle".

When considering the vocabulary of the given multiple-choice answers, choice 2: sides,mainsides, main accurately fills the blanks, as the angle formed between the two equal sides can indeed be referred to as the "main angle".

Therefore, the correct answer to the problem is: sides,mainsides, main.

Answer

sides, main

Exercise #12

In an isosceles triangle, the angle between ? and ? is the "base angle".

Step-by-Step Solution

An isosceles triangle is one that has at least two sides of equal length. The angles opposite these two sides are known as the "base angles."
The side that is not equal to the other two is referred to as the "base" of the triangle. Thus, the "base angles" are the angles between each of the sides that are equal in length and the base.
Therefore, when we specify the angle in terms of its location or position, it is the angle between a "side" and the "base." This leads to the conclusion that the angle between the side and the base is the "base angle."

Therefore, the correct choice is Side, base.

Answer

Side, base.

Exercise #13

In an isosceles triangle, the third side is called?

Step-by-Step Solution

To solve this problem, we need to understand what an isosceles triangle is and how its sides are labeled:

  • In an isosceles triangle, there are two sides that have equal lengths. These are typically called the "legs" of the triangle.
  • The third side, which is not necessarily of equal length to the other two sides, is known as the "base."

In terms of the problem, we want to determine the term used for the third side, which is the side that is not one of the two equal sides.

The correct term for the third side in an isosceles triangle is the "base." This is because the third side serves as a different function compared to the equal sides, which usually form the symmetrical parts of the triangle.

Among the given answer choices, choosing "Base" correctly identifies the third side of an isosceles triangle.

Therefore, the third side in an isosceles triangle is called the base.

Final Solution: Base

Answer

Base

Exercise #14

The sum of the adjacent angles is 180

Step-by-Step Solution

To determine if the statement that "the sum of the adjacent angles is 180" is true, follow these steps:

  • Step 1: Define Adjacent Angles

    Adjacent angles are two angles that have a common vertex and a common side but do not overlap. In geometry, when these angles form a straight line, they are known as a linear pair.

  • Step 2: Apply the Linear Pair Theorem

    The Linear Pair Theorem states that if two angles are adjacent and form a linear pair (i.e., the non-common sides form a straight line), then these angles are supplementary. This means that their sum is 180180^\circ.

  • Step 3: Conclusion

    Therefore, when adjacent angles form a linear pair on a straight line, their sum is indeed 180180^\circ.

This validates the statement that "the sum of the adjacent angles is 180" for linear pairs, making the statement True.

This corresponds to the answer choice stating: True.

Answer

True

Exercise #15

Calculate the size of angle X given that the triangle is equilateral.

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Video Solution

Step-by-Step Solution

Remember that the sum of angles in a triangle is equal to 180.

In an equilateral triangle, all sides and all angles are equal to each other.

Therefore, we will calculate as follows:

x+x+x=180 x+x+x=180

3x=180 3x=180

We divide both sides by 3:

x=60 x=60

Answer

60