Помогите решить cos(a+п/3)-cos(a-п/3)

cos(α +π/3) - cos(α - π/3) = 2*[sin(α + π/3 + α - π/3) /2]*[sin(α - π/3 -α - π/3)/2] = 2sinα*sin(-π/3) = -2*(√3/2) = - √3