Sin 2x = 2sinx*cosx
sin (π/2 - x) = cos x
домножаем числитель и знаменатель на cos π/18 (имеем право не рано 0)
( 2 * π/18 = π/9)
cos π/18 * sin π/18 * cos π/9 * cos 2π/9 / cos π/18 = sin π/9 * cos π/9 * cos 2π/9 / 2*cos π/18 = sin 2π/9 * cos 2π/9 / 4*cos π/18 = sin 4π/9 / 8*cos π/18 = sin 8π/18 / 8*cos π/18 = sin (π/2 - π/18) / 8*cos π/18 = cos π/18 / 8*cos π/18 = 1/8