Ответ:

sin(5π/12) = sin(75°) = (√6 + √2) / 4.
sin²(5π/12) = ((√6 + √2) / 4)² = (6 + 2 + 2√12) / 16 = (8 + 4√3) / 16 = (2 + √3) / 4.
√12 * sin²(5π/12) = 2√3 * (2 + √3) / 4 = √3 * (2 + √3) / 2 = (2√3 + 3) / 2.
√3 - (2√3 + 3) / 2 = (2√3 - 2√3 - 3) / 2 = -3/2.