Question 912970
You may use brackets to denote subscripts on this site, for example:
s[1] {{{ s[1] }}}
a[2k] {{{a[2k] }}}


Or if you know TeX/LaTeX, the underscore (_) is more standard:


s_1 *[tex \large s_1]
a_{2k} *[tex \large a_{2k}] (note: curly braces required for multi-char superscripts)


Back to your original question, here's a hint: draw a regular n-gon and a regular 2n-gon inside the same circle so that the vertices of the n-gon coincide with n of the vertices of the 2n-gon. You should obtain some isosceles triangles with sides *[tex \large s_n], *[tex \large s_n] and *[tex \large s_{2n}]. Do some angle chasing and perhaps apply the law of sines or law of cosines.