Show that :`cos 20^@.cos 40^@.cos 80^@=1/8`.

Prove that cos 20^@ cos 40^@ cos 60^@ cos 80^@=1/16 .

Find the value of cos 20^@ cos 40^@ cos 80^@

Without taking the help of the table of tribonometric ratios,show that:(v) sin20^@cos40^@cosec50^@sec70^@=1

Prove that sin 600^@ cos 390^@ +cos 480^@ sin 150^@ =-1.

Without taking the help of the table of trigonometric ratios,show that:(iii) sin140^@cos65^@+cos50^@cos115^@=0

Find the value of cos 130^@ + cos 110^@ + cos 10^@

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

Prove that cos 4A=1-8cos^2A+8 cos^4A

If alpha+beta=gamma ,show that cos^2alpha+cos^2beta+cos^2gamma=1+2cosalphacosbetacosgamma

The value of cos1^@cos2^@cos3^@....cos180^@ is