Prove that, `cos A+cos(120^(@)+A)+cos(120^(@)-A)=0`

Prove that : cos 10^(@) + cos 110^(@) + cos 130^(@) = 0 .

Prove that cos^3A+cos^3(120^0+A)+cos^3(240^0+A)=3/4cos3A

cos120^o=

Prove that: cos Acos(60^0-A)cos(60^0+A)=1/4cos3A

Prove that: cos(45^(@)+A) cos(45^(@)-A)=1/2 cos2A

Prove that cos65^0+cos115^0=0

Prove that cos theta + cos (120^(@) + theta) + cos ( 120^(@)-theta) =0. Hence deduce that cos ^(3) theta + cos ^(3) ( 120^(@) + theta ) + cos^3( 120^(@)-theta) = 3/4 cos 3 theta.

Prove that cos ^(2) A + cos ^(2) (A + 120^(@)) + cos ^(2) (A -120^(@)) = (3)/(2)

Prove that cos 20^(@) cos 40^(@) cos 80^(@) = (1)/(8).

Prove that: cos20^(@)cos40^(@)cos80^(@)=1/8