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Prove that : cos 20^0cos 40^0cos 60^0cos...

Prove that `: cos 20^0cos 40^0cos 60^0cos 80^0=1/(16)`

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`cos 20^(@) cos 40^(@)cos 80^(@)cos 60^(@)`
`=cos20^(@) cos (60^(@)-20^(@))cos(60^(@)+20^(@))cos60^(@)`
`=(1)/(4)cos(3xx20^(@))cos60^(@)`
`=(1)/(4)cos^(2)60^(@)=(1)/(4)xx(1)/(4)=(1)/(16)`
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