(tan(a) - tan(b))/(1+ tan(a)*tan(b)) = tan(a-b)
(tan(170o) - tan(110o))/(1+ tan(170o)*tan(110o)) = tan(170o-110o) = tan(60o) = sin(60o)/cos(60o) =
cos(a)*cos(b) + sin(a)*sin(b) = cos(a-b)
cos(5/12)*cos(/12) + sin(5/12)*sin(/12) = cos(5/12 - /12) = cos(4/12) = cos(/3) = 1/2
Comments