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Question

Find the value of cosπ5+cos2π5+cos6π5+cos7π5

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Solution

cosπ5+cos2π5+cos6π5+cos7π5

rearranging

=cosπ5+cos6π5+cos2π5+cos7π5
=cosπ5+cos(π+π5)+cos2π5+cos(π+2π5)
as cos(π+θ)=-cos θ

=cosπ5cos(π5)+cos2π5cos(2π5)=0

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