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# Lesson 22 — Activity 1: Acute Triangles

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Lesson 22 — Activity 1: Acute Triangles

#### Getting Ready

#### Before beginning this activity, click here to watch a video that explores angles within triangles in everyday life.

#### How a triangle is classified depends on the types of angles in the triangle. Remember, a triangle has three sides and three corners, so it has three angles. The angles of a triangle always add up to 180 degrees.

#### You have practised measuring angles previously in L18 — A2. Before you look at measuring angles in triangles, watch the video below as a quick review on how to measure angles using a protractor.

#### Remember from L18 — A1 that an acute angle is an angle that is less than 90 degrees. Acute triangles have three angles that are all less than 90 degrees. Look at the triangle below.

#### Notice that all three angles are less than 90 degrees. If you add the angles together, you get 180 degrees.

55 + 80 + 45 = 180

You will not always be given all of the measurements of a triangle, but you will still be asked to classify it. This is when you need to use your protractor to find the missing angles.

#### Try This:

#### Measure the unknown angle of these triangles using a protractor. Add the angles together when done. Classify each triangle by name.

#### For Triangle 1:

n = 66 degrees

180 – 66 – 48 = 66 degrees

This is an acute triangle.

For Triangle 2:

a = 47 degrees

180 – 47 – 86 = 47 degrees

This is an acute triangle.

## Digging Deeper

#### Here's an example of an acute triangle in the everyday world.

#### Can you think of other examples?

Images courtesy of www.imagesgoogle.com