1 + Cos8x = 2Cos4x
2Cos²4x - 2Cos4x = 0
Cos²4x - Cos4x = 0
Cos4x(Cos4x - 1) = 0
1) Cos4x = 0
2) Cos4x - 1 = 0
Cos4x = 1
1 + Cos8x = 2Cos4x
2Cos²4x - 2Cos4x = 0
Cos²4x - Cos4x = 0
Cos4x(Cos4x - 1) = 0
1) Cos4x = 0
2) Cos4x - 1 = 0
Cos4x = 1