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cospi/5+cos(2pi)/5+cos(6pi)/5+cos(7pi)/5...

`cospi/5+cos(2pi)/5+cos(6pi)/5+cos(7pi)/5=0`

Text Solution

Verified by Experts

`cos""(pi)/(5)+cos""(2pi)/(5)+cos""(6pi)/(5)+cos""(7pi)/(5)`
`=(cos""(pi)/(5)+cos""(7pi)/(5))+(cos""(2pi)/(5)+cos""(6pi)/(5))`
`=2cos""(4pi)/(5)cos""(3pi)/(5)+2cos""(4pi)/(5)cos"" (2pi)/(5)`
`=2cos""(4pi)/(5)(cos""(3pi)/(5)+cos""(2pi)/(5))`
`=2cos""(4pi)/(5)(2cos""(pi)/(2)cos""(pi)/(10))=0`
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