## Content description

Demonstrate that the angle sum of a triangle is 180° and use this to find the angle sum of a quadrilateral (ACMMG166)

### Elaborations

• using concrete materials and digital technologies to investigate the angle sum of a triangle and quadrilateral

Source: Australian Curriculum, Assessment and Reporting Authority (ACARA)

### 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.

1. Cut out a triangle. Tear the corners off and place them together so that they form a straight angle.

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2. Draw a number of different triangles. Measure the angles and check that their sum is 180°. Set squares are excellent examples of triangles.

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3. Cut out a triangle. Fold it without tearing it to demonstrate that the three interior angles form a straight angle.

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This third demonstration is close to a proof.