This is exercise 21.3, of <A HREF="http://www.amazon.com/dp/0821843168/ref=rdr_ext_tmb">Mathematical Omnibus: Thirty Lectures on Classic Mathematics</A>, by D.B. Fuks and Serge Tabachnikov. The solution is given on page 448:

![Solution of problem 21.3 from Mathematical Omnibus][1]


  [1]: https://i.sstatic.net/iHu7o.png