Exterior Angles of Triangles Practice Problems & Solutions

Master exterior angle theorems with step-by-step practice problems. Learn to find missing angles using the exterior angle property of triangles.

📚Practice Exterior Angle Problems and Build Your Confidence
  • Apply the exterior angle theorem to find missing interior angles
  • Calculate exterior angles using the sum of non-adjacent interior angles
  • Solve problems involving supplementary relationships between adjacent angles
  • Work with right triangles and exterior angle properties
  • Use the 360° sum property of all exterior angles
  • Practice both direct calculation and algebraic methods

Understanding Exterior angles of a triangle

Complete explanation with examples

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

Detailed explanation

Practice Exterior angles of a triangle

Test your knowledge with 64 quizzes

Determine the size of angle ABC?

DBC = 100°

DDDBBBCCCAAA10040

Examples with solutions for Exterior angles of a triangle

Step-by-step solutions included
Exercise #1

Determine the type of angle given.

Step-by-Step Solution

To solve this problem, we'll examine the image presented for the angle type:

  • Step 1: Identify the angle based on the visual input provided in the graphical representation.
  • Step 2: Classify it using the standard angle types: acute, obtuse, or straight based on their definitions.
  • Step 3: Select the appropriate choice based on this classification.

Now, let's apply these steps:

Step 1: Analyzing the provided diagram, observe that there is an angle formed among the segments.

Step 2: The angle is depicted with a measure that appears greater than a right angle (greater than 9090^\circ). It is wider than an acute angle.

Step 3: Given the definition of an obtuse angle (greater than 9090^\circ but less than 180180^\circ), the graphic clearly shows an obtuse angle.

Therefore, the solution to the problem is Obtuse.

Answer:

Obtuse

Video Solution
Exercise #2

Indicates which angle is greater

Step-by-Step Solution

Note that in drawing B, the two lines form a right angle, which is an angle of 90 degrees:

While the angle in drawing A is greater than 90 degrees:

Therefore, the angle in drawing A is larger.

Answer:

Video Solution
Exercise #3

Indicates which angle is greater

Step-by-Step Solution

In drawing A, we can see that the angle is an obtuse angle, meaning it is larger than 90 degrees:

While in drawing B, the angle is a right angle, meaning it equals 90 degrees:

Therefore, the larger angle appears in drawing A.

Answer:

Video Solution
Exercise #4

Which angle is greater?

Step-by-Step Solution

The angle in diagram (a) is more acute, meaning it is smaller:

Conversely, the angle in diagram (b) is more obtuse, making it larger.

Answer:

Video Solution
Exercise #5

Indicates which angle is greater

Step-by-Step Solution

Answer B is correct because the more closed the angle is, the more acute it is (less than 90 degrees), meaning it's smaller.

The more open the angle is, the more obtuse it is (greater than 90 degrees), meaning it's larger.

Answer:

Video Solution

Frequently Asked Questions

What is the exterior angle theorem for triangles?

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The exterior angle theorem states that an exterior angle of a triangle equals the sum of the two non-adjacent interior angles. For example, if angles A and B are 50° and 30°, the exterior angle at vertex C would be 80°.

How do you find an exterior angle of a triangle step by step?

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To find an exterior angle: 1) Identify the two interior angles that are NOT adjacent to the exterior angle, 2) Add these two angles together, 3) The sum equals the exterior angle. Alternatively, subtract the adjacent interior angle from 180°.

What is the sum of all exterior angles of a triangle?

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The sum of all three exterior angles of any triangle is always 360°, regardless of the triangle's shape or size. This is a fundamental property that applies to all triangles.

How are exterior and interior angles of triangles related?

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An exterior angle and its adjacent interior angle are supplementary, meaning they add up to 180°. The exterior angle also equals the sum of the two remote (non-adjacent) interior angles.

What are common mistakes when solving exterior angle problems?

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Common errors include: confusing which angles are adjacent vs. non-adjacent, forgetting that exterior and adjacent interior angles sum to 180°, and incorrectly identifying which angle is actually the exterior angle.

Can you use exterior angles to find interior angles of triangles?

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Yes! If you know an exterior angle, you can find interior angles by: 1) Subtracting from 180° to get the adjacent interior angle, or 2) Using the exterior angle theorem if you know one of the non-adjacent interior angles.

Do exterior angle rules work for all types of triangles?

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Yes, the exterior angle theorem applies to all triangles - acute, right, obtuse, scalene, isosceles, and equilateral. The relationships between exterior and interior angles remain consistent regardless of triangle type.

How do you identify an exterior angle in a triangle diagram?

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An exterior angle is formed between one original side of the triangle and the extension of an adjacent side. It lies outside the triangle and is supplementary to the interior angle at the same vertex.

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