Is there an easy way to prove the identity?
cos(π7)−cos(2π7)+cos(3π7)=cos(π3)
While solving one question, I am stuck, which looks obvious but without any feasible way to approach.
Few observations, not sure if it would help
π7+3π72=2π7π7+3π7+2π7=π−π7
Answer
Yes, This problem in 1963 IMO.http://www.artofproblemsolving.com/Forum/viewtopic.php?p=346908&sid=8ad587e18dd5fa9dd5456496a8daadfd#p346908
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