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Prove that: cos20^0cos40^0cos60^0cos80^0...

Prove that: `cos20^0cos40^0cos60^0cos80^0=1/(16)`

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  3. Prove that: cos20^0cos40^0cos60^0cos80^0=1/(16)

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  4. Prove that: sin20^0sin40^0sin60^0sin80^0=3/(16)

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