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.