2.8 Решить: cos(π/12) * cos(2π/3) - sin(π/12) * sin(2π/3)

Используем формулу косинуса суммы углов: cos(a + b) = cos(a)cos(b) - sin(a)sin(b). В нашем случае a = π/12, b = 2π/3. cos(π/12 + 2π/3) = cos(π/12 + 8π/12) = cos(9π/12) = cos(3π/4) = -√2/2. Ответ: -√2/2