Let a = cos^(-1) cos 20, b = cos^(-1) cos 30 and c = sin^(-1) sin (a + b) then If 5 sec^(-1) x + 10 sin^(-1) y = 10 (a + b + c) then the value of tan^(-1) x + cos^(-1) (y -1) is
Prove that cos20.cos40.cos60.cos80=1/19
Prove that cos80+cos40-cos20=0
The value of cos 20° + cos 40° + cos 60° +......cos 160° + cos 180° is