Question

$\cos {10}^o+\cos {20}^o+....+\cos {170}^o$.

Answer 1

${\cos 10}^0+{\cos 20}^0+......{\cos 170}^0$

$={\cos 10}^0+{\cos 120}^0+......{\cos 10}^0$

$={\cos 90}^0$

$=0$