Show that`cos24^@+cos55^@+cos125^@+cos204^@+cos300^@=1/2`

cos20^@cos40^@cos80^@=

cos1^@+cos2^@+cos3^@+...+cos180^@=

1+cos56^@+cos58^@-cos66^@=

Show that cos510^@cos330^@-sin390^@cos120^@=-1/2

Without using trignometric tables,prove the following: cos40^@+cos50^@+cos70^@+cos80^@=cos20^@+cos10^@

Prove the following: cos20^@cos40^@cos60^@cos80^@=1/16

The value of cos52^@+cos68^@+cos172^@ is

1+ cos 10^(@) + cos 20^(@)+ cos 30^(@)= ............... A) 4 cos 5^(@) cos 10^(@) cos 15^(@) B) 4 cos 10^(@) cos 20^(@) cos 30^(@) C) 4 sin 5^(@) sin 10^(@) sin 15^(@) D) 4 sin 10^(@) sin 20^(@) sin 30^(@)

The value of cos 52^@ + cos 68^@ + cos 172^@