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Prove that: cos 80^0+cos 40^0-cos 20^0=0...

Prove that: `cos 80^0+cos 40^0-cos 20^0=0`

Text Solution

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`cos80^(@)+cos40^(@)-cos20^(@)`
`=2cos((80^(@)+40^(@))/(2))cos((80^(@)-40^(@))/(2))=cos20^(@)`
`=2cos60^(@)cos20^(@)-cos20^(@)`
`=2(1)/(2)cos20^(@)-cos20^(@)=0`
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