Cos20•cos40•cos60•cos80=1/16

Use: cos @× cos (60-@)×cos (60+@) =cos 3@/4 and cos 60= 1/2 Therefore cos 20.cos40.cos60.cos80 =cos 20 cos (60-20) cos (60+30) cos60 = cos 60.cos60/4 = 1/2×1/2×1/4 =1/16

2Thank You

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