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Prove that cos10^@cos30^@cos50^@cos70^@=...

Prove that `cos10^@cos30^@cos50^@cos70^@=3/(16)`.

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  12. Prove thatsin10^@sin50^@sin60^@sin70^@=sqrt3/(16)

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  13. Prove that tan20^@tan30^@tan40^@tan80^@=1

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