This is also from The IrrationalsÂ – though I had to ask for assistance over at Stack Exchange to get the right answer (as is so often the case the solution is reasonably obvious when you are presented with it).

Anyway, the question is: given a regular pentagon (of sides with length of 10 units) which is inscribed by a circle, what is the diameter of the circle?

This figure helps illustrate the problem:

We are trying to find and we know that .

If we knew then we could answer as . We do not know that but we do know that and hence, from the identity: .

From our knowledge of the pentagon with sides of unit length (you’ll have to trust me on this or look it up – it’s too much extra to fit in here) we also know that , where is the golden ratio.

Hence … well, the rest is left as an exercise for the reader đź™‚