The following problem is from the 18th Balkan Mathematics Olympiad.
"In a pentagon all interior angles are congruent and all its sides have
rational lengths. Prove that this pentagon is regular."
Besides the fact that no generality is lost replacing rational sides by integer sides, I am totally lost on this one. I would like hints only, as small as you can make them. I would like to come to a solution on my own if possible.
Best Answer
I guess I'd try representing the space spanned by unit vectors in the five relevant directions as a $\mathbb Q$-vectorspace. Its dimension should be $4$, so there exists a non-trivial linear combination which evaluates to zero (i.e. which closes the pentagon), and the coefficients for that linear combination should be all equal, corresponding to unit length.
Detais (hidden inside a spoiler block, so move mouse over to read):