Тут надо применит формулы суммы( разности) тригонометрических функций
1)2Сos(5x/4 + π/8)Сos(3x/4 - π/8) = 0
Cos(5x/4 +π/8) = 0 или Cos(3x/4 - π/8) = 0
5x/4 +π/8 = π/2 +πk, k Є Z 3x/4 - π/8 = π/2 + πk , k ЄZ
5x/4 = π/2 - π/8 +πk, k Є Z 3x/4 = π/2 + π/8 + πk , kЄ
x =4π/10 - 4π/40 +4πk/5, kЄ Z x = 4 π/6 + 4π/24 + 4πk /3, kЄ
x = 2π/5 - π/10 +4πk/5, k Є Z x = 2π/3 + π/6 + 4πk /3, k ЄZ
2) 2Cosπ/4Cos x = 1
2*корень(2)/2*Cosx = 1
Cosx = 1/корень(2)
x = +-π/4 +2πk, k Є Z
3) 2Sinx Cos π/6 = 1
Sinx* корень(3) = 1
Sin x = 1/корень(3)
x = (-1)^n arcSin1/корень(3) + nπ, n Є Z