Question

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

Answer 1

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]`