Stage: 5 Challenge Level: 
Why do this problem?
A well known result but nevertheless a good exercise in using vectors including the scalar product.
Key question
How do you write down what is given in the question in terms of vectors?
Tetrahedra. Pythagoras' theorem. Vector geometry. Quadratic equations. Long problems. epsilons. Vector algebra. Index notation/Indices. Vector notation. Calculating with ratio & proportion.
Published June 1998.
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