Help me evaluate : cos (π/7) - cos (2π/7) + cos (3π/7) ?

1 Answer
Nghi N.
Sep 19, 2015

Evaluate : cos ((pi)/7) - cos ((2pi)/7) + cos ((3pi)/7)

Explanation:

First, evaluate the sum:
cos ((pi)/7) + cos ((3pi)/7) = 2cos ((2pi)/7)cos ((pi)/7)
Next, evaluate:
2cos ((2pi)/7).cos ((pi)/7) - cos ((2pi)/7) =
= cos ((2pi)/7)[2cos (pi/7) - 1]

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