Stage: 3 Challenge Level: 
ABCD is a regular tetrahedron and the points P, Q, R and S are the midpoints of the edges AB, BD, CD and CA. Prove that PQRS is a square.
Mathematical reasoning & proof. Interactivities. Generalising. Tetrahedra. Working systematically. smartphone. Games. Cubes. Squares. Visualising.
Published September 2001.
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