1. The problem statement, all variables and given/known data Write the following in terms of the cofunction identity: cos(4pi/3) 2. Relevant equations I know the θ is in Quadrant 3, but my question is, which cofunction identity am I suppose to use? Cofunction Identities: cosx = sin(pi/2 - x) #1 cosx = sin(pi/2 + x) #2 cos(pi + x) = -cosx cosx = -cos(pi + x) #3 cos(pi - x) = -cosx cosx = -cos(pi - x) #4 Is #2 only applicable for angles in quadrant 2 in terms of cofunction identity? But I tried using it and it still gave me the trig ratio. So what is the difference? 3. The attempt at a solution #1 cos(4pi/3) = sin(pi/2 - 4pi/3) = -0.5 #2 cos(4pi/3) = sin(pi/2 + 4pi/3) = -0.5 #3 cos(4pi/3) = -cos(pi + 4pi/3) = -0.5 #4 cos(4pi/3) = -cos(pi - 4pi/3) = -0.5 See, as shown, they all work. I have seen the triangles and know how they are derived, but my main question is, if I am given a question that asks for the equivalent trig ratio in terms of its cofunction identity, is there a specific one I should use when I am doing these types of question? Your help is much appreciated.