The value of `co s (2pi) / 21 + co s (4pi) / 21 + co s (6pi) / 21 + .....+ co s (20pi) / 21` is equal to

The value of cos(2 pi)/(21)+cos(4 pi)/(21)+cos(6 pi)/(21)+....+cos(20 pi)/(21) is equal to

The value of cos((2pi)/7)cos((4pi)/7)cos((6pi)/7) is

Prove that cos ((2pi)/7)+ cos ((4pi)/7) + cos ((6pi)/7) = - 1/2

Prove that cos ((2pi)/7)+ cos ((4pi)/7) + cos ((6pi)/7) = - 1/2

The value of cos (pi)/(5) . cos (2 pi)/(5). cos (4 pi)/(5) . cos (8 pi)/(5) is

If P = cos ((pi) / (19)) + cos ((3 pi) / (19)) + cos ((5 pi) / (19)) + ... + cos ((17 pi) / ( 19)) and Q = cos ((2 pi) / (21)) + cos ((4 pi) / (21)) + ... cos ((20 pi) / (21)) then find PQ