Question

Prove that :- cos (π/15) + cos (4π/15) + cos (11π/15) + cos (14π/15) = 0​

Answer 1

Answer:

Please see the attachment

Answer 2

cos (π/15) + cos (4π/15) + cos (11π/15) + cos (14π/15) = 0

Step-by-step explanation:

cos (π/15) + cos (4π/15) + cos (11π/15) + cos (14π/15) = 0

LHS =

cos (π/15) + cos (4π/15) + cos (11π/15) + cos (14π/15)

= cos (π/15) + cos (4π/15) + cos (π - 4π/15) + cos (π - π/15)

now using the concept

cos x = - cos (π - x)

= cos (π/15) + cos (4π/15) +- cos (4π/15) - cos (π/15)

= 0

= RHS

QED

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