LET’S TRY ONE
What is the sum in degrees of the interior angles of a 5-sided simple convex polygon?
540
Draw diagonals from one vertex to form 3 triangles. Their angles total 3 × 180 = 540°.
Learning steps & teaching tips
Use a triangle decomposition to find an angle sum.
- A convex n-sided polygon can be divided into n−2 triangles from one vertex.
- Work through the example and explain why each operation is used.
- Estimate or use another method to check the result before choosing.
Try it together
Work with a partner: solve one example, explain the method, then change one number and predict what changes.
A convex n-sided polygon can be divided into n−2 triangles from one vertex.