1/2 + (-1/2) = 0
4*( 2 sin 7π/6) = 2 * ( 2 * (-1/2) ) = -2
по формулам суммы
(2cos60/2*cos24/2) / cos12 =( 2cos30*cos12)/cos12 = 2cos30 = 2*√3/2=√3
