Решение
1) cos75 = cos(15+60)
Cos15 + cos(15+60) = -2*sin (15+69+15)/3*sin(15+60-15)/2 = -2*sin45*sin39= -2*(1/2)*(√2 /2) - √2 /2
-√2 / 2 + √2 / 2 =0
2 sinα - sin (α +2π/3) = 2sin((α- α- α2π/3))/2 * cos((2α+2π/3))/2 = 2sin π/3 *cos (α +α/3) = √3 cos(π/3 +α)