Cos5a*cos4a-sin2a*cos2a
Cos5a*cos4a-sin2a*cos2a= cos5a*cos4a - sin2a*cos2a * 2 * 1/2 = cos5a*cos4a-1/2 * sin4a= cos5a*cos4a-sin5a*sin4a = cos(5a+4a) = cos9a