Question

Buktikan cos pi/7 + cos 2pi/7 + cos3pi/7 = 1

Answer 1

Cos π/7+ cos 2π/7+ cos 3π/7=1... cos 6π/7=1»pke identitas ya

Answer 2

Jawabannya : 1/2

cos (3π / 7) - cos (2π / 7) + cos (π / 7)
=> Cos (3π / 7) - cos (π - 5π / 7) + cos (π / 7)
=> Cos (3π / 7) + cos (5π / 7) + cos (π / 7)
=> Cos (π / 7) + cos (3π / 7) + cos (5π / 7). . . {Menata ulang ketentuan}

[1 / 2sin (π / 7)] [2sin (π / 7) cos (π / 7) + 2sin (π / 7) cos (3π / 7) + 2sin (π / 7) cos (5π / 7)]
=> [1 / 2sin (π / 7)] [sin (2π / 7) + sin (4π / 7) - sin (2π / 7) + sin (6π / 7) - sin (4π / 7)]
=> [1 / 2sin (π / 7)] [sin (6π / 7)]
=> [1 / 2sin (π / 7)] [sin (π - π / 7)]
=> [1 / 2sin (π / 7)] [sin (π / 7)]
=> 1/2 =>  (tidak Terbukti)