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Prove that: i) cos10^(@)cos30^(@)cos50...

Prove that:
i) `cos10^(@)cos30^(@)cos50^(@)cos70^(@)=3/16`
ii) `cos20^(@)cos40^(@)cos60^(@)cos80^(@)=1/16`
iii) `4cos12^(@)cos48^(@)cos72^(@)=cos36^(@)`
iv) `cos40^(@) cos80^(@)cos160^(@)=-1/8`

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NAGEEN PRAKASHAN ENGLISH-TRIGNOMETRIC FUNCTIONS-EXERCISES 3E
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