Investigating the interior angles of a triangle
The first important result about a triangle is that the sum of its three interior angles is always 180°. There are a number of ways to demonstrate this result.
- Cut out a triangle. Tear the corners off and place them together so that they form a straight angle.
- Draw a number of different triangles. Measure the angles and check that their sum is 180°. Set squares are excellent examples of triangles.
- Cut out a triangle. Fold it without tearing it to demonstrate that the three interior angles form a straight angle.
This third demonstration is close to a proof.