1. The problem statement, all variables and given/known data Find the cosines of the internal angles of the triangle which has the following coordinate vectors as its vertices: A(4,0,2) B(6,2,1) C(5,1,6) 2. Relevant equations I understand that you have to find the 3 vectors which represents each side of the triangle in order to get the cosine. My problem is, which vectors do I use for each angle? Lets say that I want to find the cosine of the angle at vertex B. Do I use vector AB and BC or do I use BA and BC? When I use AB(b-a=(2,2,-1)) and BC(c-b=(-1,-1,5)) the dot product of the vectors are negative whereas when I use BA and BC its positive hence I get different answers. Which one do I use and why?