1.) Move the vertices of the triangle to determine the shape and size of the triangle you want.

2.) Move the slider to the extreme right. What do you observe?

3.) What is the relationship between the three angles which meet at a point, and the three interior angles of a triangle?

4.) What can you say about the angle sum of the parallelograms/trapezium formed?

5.) Make a conjecture about your observation.

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Another demonstration that shows that the sum of the interior angle of a triangle is 180°.

2.) Move the slider to the extreme right. What do you observe?

3.) What is the relationship between the three angles which meet at a point, and the three interior angles of a triangle?

4.) What can you say about the angle sum of the parallelograms/trapezium formed?

5.) Make a conjecture about your observation.

Another demonstration that shows that the sum of the interior angle of a triangle is 180°.