Rectangle

This shape has been constructed so that when the vertices A, B and C are moved, it is always a rectangle.
Try moving these vertices, what changes? What stays the same?

Now draw the diagonals by selecting the segment tool and left-clicking on A and C and then on B and D.
Select the point tool and put a point, E, on the intersection point of the two diagonals.

Use the distance tool to check that for any rectangle, the diagonals are the same length and they bisect each other.

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Acknowledgements: http://www.lsp.open.ac.uk/geometry/

Created with GeoGebra