$\begin{array}{*{20}{c}}{\frac{{\sin 0 + \sin 1 + \sin 2 + .......... + \sin n}}{{\cos 0 + \cos 1 + \cos 2 + ......... + \cos n}}}\\\parallel \\{\tan (?)}\end{array}$

L.H.S. $ = \frac{{\frac{{\sin \left( {0 + \frac{{n + 1 - 1}}{2}.1} \right)\sin \frac{{(n + 1).1}}{2}}}{{\sin \frac{1}{2}}}}}{{\frac{{\cos \left( {0 + \frac{{n + 1 - 1}}{2}.1} \right)\sin \frac{{(n + 1).1}}{2}}}{{\sin \frac{1}{2}}}}}$ 

(note that there are n+1 terms in both the numerator and the denominator)

$ = \frac{{\sin \frac{n}{2}}}{{\cos \frac{n}{2}}} = \tan \frac{n}{2}$

$\therefore \, ? = \frac{n}{2}$