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Find the value of cos 12^(@)+cos 84^(@)+...

Find the value of `cos 12^(@)+cos 84^(@)+cos 156^(@)+cos 132^(@)`

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`cos 12^(@)+cos 84^(@)+cos 156^(@)+cos 132^(@)`
`=(cos 12^(@)+cos132^(@))+(cos 84^(@)+cos156^(@))`
`=2cos((12^(@)+132^(@))/(2))cos((132^(@)-12^(@))/(2))`
`+2cos((84^(@)+156^(@))/(2))cos((156^(@)-84^(@))/(2))`
`=2cos 72^(@)cos 60^(@)+2cos 120^(@)cos 36^(@)`
`=2sin 18^(@)cos 60^(@)+2cps 120^(@)cos 36^(@)`
`=2((sqrt(5)-1)/(4))(1)/(2)+2(-(1)/(2))((sqrt(5)+1)/(4))=-(1)/(2)`
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