Посчитаем отдельно, что получится в скобках:
3π/2 + α = 3π/2 + 7π/6 = (3*3 + 7)π/6 = 16π/6 = 8π/3
cos(8π/3) = cos(2π/3) = cos(π/3) = 1/2
2π - a = 2π - 7π/6 = (2*6 - 7)π/6 = 5π/6
sin(5π/6) = sin(π/6) = 1/2
Собираем всё воедино:
(5*1/2) / (4*1/2) = 5/4 = 1,25