Sin4x+cos18x = 0

Помогите решить

Sin4x + sin18x = 0; [ sin18x+sin4x = 0 ].

2sin (4x+18x) / 2*cos (4x-18x) 2 = 0;

2≠ 0;

sin11x*cos7x=0, [ cos (-α) = cosα ];

sin11x=0 ⇒ 11x = π*k; x = (π/11) * k;

cos7x = 0 ⇒7x = π/2 + π*k; x = π/14 + (π/7) * k; k - целое число.