(Cos 5x + Cos 2x) + ( Cos3x + Cos 4x) = 0
2Cos3,5xCos1,5x + 2Cos3,5xCos0,5x=0
2 Cos3,5x(Cos1,5x+Cos0,5x) = 0
2Cos 3,5x·2CosxCos0,5x = 0
Cos 3,5x = 0 или Cos x = 0 или Cos0,5x = 0
3,5х = π/2 + πк, к ∈Z; х = π/2 + πn, n ∈Z; 0,5x = π/2 + πm,m∈Z
х =π/7 +7πк/2,к∈Z x =π + 2πm,m∈Z