hasil dari cos 2x=cos Π/4 , dengan 0≤x≤2Π

cos 2x = cos π/4
2x = π/4 + k . 2π atau 2x = -π/4 + k . 2π
x = π/8 + k . π .................... x = -π/8 + k . π
x = π/8 ................................. x = -π/8 (tdk memenuhi) => utk k = 0
x = 9π/8 ............................... x = 7π/8 => utk k = 1
x = 17π/8 (tdk memenuhi) .. x = 15π/8 => utk k = 2
HP = {π/8, 7π/8, 9π/8, 15π/8}

Cos2x=cos 180°/4
cos2x= cos45°
Cos2x= ½√2