Cos(a+b) = cos(a)*cos(b) - sin(a)*sin(b);
cos(a-b) = cos(a)*cos(b) + sin(a)*sin(b);
складываем последние два уравнения,
cos(a+b) + cos(a-b) = 2*cos(a)*cos(b);
cos(a)*cos(b) = (1/2)*( cos(a+b) + cos(a-b) );
cos(285*)*co(165*) = (1/2)*( cos(285*+165*) + cos(285*-165*) ) =
= (1/2)*( cos(450*) + cos(120*) ) = (1/2)*( cos(360*+90*) + cos(180*-60*) ) = (1/2)*(cos(90*) - cos(60*) ) = (1/2)*( 0 - (1/2)) = -1/4 = -0,25.
(350+210):(350-210)=560:140=4
Ответ: 4
Разделяем на два
√3/2sinx-1/2cosx=1/2
sinx*cos(π/6)-sin(π/6)cosx=1/2
sin(x-π/6)=1/2
x-π/6=(-1)^k*π/6+πk,k∈Z
x=(-1)^k*π/3+πk,k∈Z