The angle sum of a quadrilateral

ABCD is a quadrilateral. Click and drag any of the blue points to change the shape of the quadrilateral.

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1. What observation can you make regarding the angle sum of a quadrilateral?
Move the slider to the right.
2. What relationship exists between the angle sum of a triangle and the angle sum of a quadrilateral?
Move the point B to a position so that <ABC is a reflex angle.
3. What is the special name given to such a polygon?

State of New South Wales, Department of Education and Training 2007, Created with GeoGebra