Urg, really struggling with this one! Any help i'd really appreciate!

Show that(where Q is theta =) )

$\displaystyle

\frac{e^{nQi} - 1}{e^{Qi} - 1} = \frac{sin(\frac{Qn}{2})}{sin(\frac{Q}{2})}e^\frac{ (n-1)Qi}{2}

$

provided that Q is not a multiple of 2pi...

Thanks!!