Prove that :
`cos 20^@ + cos 100^@ + cos 140^@ = 0`

Prove that : sin 105^@ + cos 105^@ = cos 45^@ .

Prove that : cos 24^@+ cos 55^@ + cos 125^@ + cos204^@ + cos 300^@= 1/2 .

Prove that: cos20^@+cos100^@+cos140^@=0 .

Prove that : cos 130^@ cos 40^@+ sin 130^@ sin 40^@= 0 .

If A, B, C and D be the angles of a cyclic quadrilateral, taken in order, prove that : cos (180^@- A) + cos (180^@+ B) + cos (180^@+ C)- sin (90^@ + D) = 0 .

Prove the following : cos 20^@ cos 40^@ cos 80^@ =1/8 .

Prove that: sin 150^@ cos 120^@+ cos 330^@ sin 660^@= - 1 .

Prove the following : cos 10^@ cos 50^@ cos 70^@ = sqrt3/8 .

Prove that : cos^2 30^@, cos^2 45^@, cos^2 60^@ are in A.P.

Prove the following : cos 20^@ cos 40^@ cos 60^@ cos 80^@ =1/16 .