cos 2pi/7 + cos 4pi/7 + cos 6pi/7 +..............is

a).is eqal to zero , b).is a negative no, c).lies between 0 and 3 , d). lies between 3 and 6

According to me, it should be a negative number because if you look into the interval of  more values of cos are negative than values which are positive.

Hope it helps!