1) Воспользуемся формулами:
cos(x + y) = cosx * cosy - sinx * siny;
cos(x - y) = cosx * cosy + sinx * siny;
cos(x - 30°) - cos(x + 30°) = 0;
cosx * cos30° + sinx * sin30° - cosx * cos30° + sinx * sin30° = 0;
2 * sinx * sin30° = 0;
sinx = 0;
x = πk, k ∈ Z;