Angles and triangles

Ages 9 to 12 Read alone or together · about 25 minutes

Read together

An angle measures a turn. A right angle is a quarter turn, 90 degrees.

The three angles of any triangle add up to 180 degrees.

Do it

1. Find right angles in a room: a corner of a book, a door.
2. Draw a triangle, cut off its three corners and place them side by side. They form a straight line.
3. Measure the angles with a protractor and add them.
4. Try a different triangle. Is the total the same?

Talk about it

• What do the three angles add up to?
• Is a corner of a page a right angle?

For the grown-up

The angle sum of a triangle is 180 degrees on a flat surface. Seeing it by tearing the corners builds belief before proof. Use safe scissors.

Mark as done

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Where this comes from

• National Governors Association Center for Best Practices and Council of Chief State School Officers, Common Core State Standards for Mathematics (2010), standard 4.MD.C (angle measurement) and 8.G.A.5 (angle sum of triangles)
• Euclid, Elements, Book I, Proposition 32

Written with AI assistance and checked against the sources listed. Please read it before sharing it with a child.