### Why use this
problem?

The problem introduces an attractive generalisation of Pythagoras'
theorem to 3D and gives good practice in the use of the area
formula for a triangle and the cosine rule. It involves the use of
Pythagoras' theorem in 2D and some algebraic manipulation.

An entirely different approach to solving the problem involves
vectors.

### Possible approach

Pre-planning of how to tackle the problem involves asking:

### Key questions

What information is given?

What can be deduced from the geometrical properties?

What are the unknowns and what notation shall we use?

What equations can we write down for the areas of the four
triangular faces of the tetrahedron?

What equations can we write down involving the lengths of the edges
of the tetrahedron?

How can we put all this together to get the required result?

### Possible support

Try the problem

Rectangular Pyramids.
### Possible extension

Prove the result using vectors.

Generalise the result to a cosine rule for a tetrahedron.