Cos 12 x*cos7x-cos4x*cos 15 x=0

   1. Творение косинусов:

• 2cosx * cosy = cos(x + y) + cos(x - y);

• cos12x * cos7x - cos4x * cos15x = 0;
• 2cos12x * cos7x - 2cos4x * cos15x = 0;
• cos(12x + 7x) + cos(12x - 7x) - (cos(15x + 4x) + cos(15x - 4x)) = 0;
• cos19x + cos5x - cos19x - cos11x = 0;
• cos5x - cos11x = 0.

   2. Разность косинусов:

• cosx - cosy = 2sin((y + x)/2) * sin((y - x)/2);

• 2sin((11x + 5x)/2) * sin((11x - 5x)/2) = 0;
• 2sin8x * sin3x = 0;

• [sin8x = 0;
  [sin3x = 0;
• [8x = k, k Z;
  [3x = k, k Z;
• [x = k/8, k Z;
  [x = k/3, k Z.

   Ответ: k/8; k/3, k Z.