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Properties of Triangles - Interior/Exterior Angles

Author:Danielle Williams

Quick Review:

Please classify the triangles below by their sides and angles. DO NOT move any of the points.

Triangle ABC is an

Select all that apply
  • A
  • B
  • C
  • D
Check my answer (3)

Triangle DEF is an

Select all that apply
  • A
  • B
  • C
  • D
Check my answer (3)

Triangle GHI is an

Select all that apply
  • A
  • B
  • C
  • D
Check my answer (3)

Task One

Below you will see a triangle created with all of its interior angles measured. Use this triangle to answer the following questions:

Add all three of the interior angles together (including the decimals). What did you get as your answer?

Move point B and add all of them together again. What did you get this time?

What statement can we make about the interior angles of any triangle?

Task Two

Use the diagram below to answer the following questions:

1. What is the measure of the third angle? Hint: Imagine that (2x - 12) is not there.

2. Find the value of x.

Task Three

Below you will see a triangle created with two of its interior angles measured and one of its exterior angles measured. Use this triangle to answer the following questions:

Add together the angle measures at point A and point B. How does this relate to the measure of the exterior angle?

Move point B. What do you notice about the relationship between the two interior angles and the exterior angle?

What statement can we make about the relationship between the two interior angles and its non-adjacent (not next to) exterior angle?

Task Four

Use the diagram below to answer the following questions:

1. Find y.

2. How did you find the value of y?

3. Is there another way to find y?

Task Five

Below you will see a triangle created with all of its interior angles measured with one of its interior angles being 90. Use this triangle to answer the following questions:

Add together the angles at point D and point C. What did you get?

Move point C. What do you get when you add together the angles at point D and point C this time?

What statement can we make about the sum (adding together) of the other interior angles when one of the interior angles is already 90?

Task Six

Use the diagram below to answer the following question:

1. Find z.

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