cos 9x cos 7x - sin 9x sin 7x = - 1\2

cos(9x)cos(7x) - sin(9x)sin(7x) = -1/2

1/2*(cos(2x) + cos(16x)) - 1/2(cos(2x) - cos(16x)) = -1/2 // Преобразовали произведение

1/2*cos(16x) + 1/2*cos(16x) = -1/2

cos(16x) = -1/2

x = π/24 + πk/8

x = -π/4 + πk/8

Ответ: {-π/4 + πk/8; π/24 + πk/8, k - целое}