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Question Number 168297 by MdNafiz last updated on 07/Apr/22
provethat........cos2Π7+cos4Π7+cos8Π7=−12
Answered by som(math1967) last updated on 07/Apr/22
letπ7=θ∴cos2θ+cos4θ+cos8θ=2sin2θcos2θ+2sin2θcos4θ+2sin2θcos8θ2sin2θ=sin4θ+sin6θ−sin2θ+sin10θ−sin6θ2sin2θ=sin4θ+sin10θ−sin2θ2sin2θ=2sin7θcos3θ−sin2θ2sin2θ=0−sin2θ2sin2θ=−12★★sin7θ=sinπ=0
Commented by peter frank last updated on 08/Apr/22
good
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