Square-Bracing Problem

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Classic Problem

Let there be an unlimited supply of rigid rods of unit length which may be joined together at their ends with a hinged joint.

Let $4$ of these rods be used to construct a square.


How many more of these rods are needed to make the square rigid?


Solution

Non-Crossing Rods

This solution uses $23$ rods:


BracedSquareUncrossing.png





Crossing Rods

This solution uses $15$ rods:


BracedSquareCrossing.png





Sources