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Prove that sin 20^(@) sin 40^(@) sin 60^...

Prove that `sin 20^(@) sin 40^(@) sin 60^(@) sin 80^(@)=3/(16)`.

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`sin20^(@)sin40^(@)sin60^(@)sin80^(@)`
`=sin60^(@)sin20^(@)sin(60^(@)-20^(@))sin(60^(@)+20^(@))`
`=(1)/(4)sin60^(@)sin(3xx20^(@))=(1)/(4)sin^(2)60^(@)=(3)/(16)`
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