Four rods are hinged at their ends to form a convex quadrilateral.
Investigate the different shapes that the quadrilateral can take.
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What can you say about the lengths of the sides of a quadrilateral whose vertices are on a unit circle?
Can you prove this formula for finding the area of a quadrilateral from its diagonals?
Someone showed me this picture of a quadrilateral made by
joining together five smaller quadrilaterals.