1. The problem statement, all variables and given/known data ABCD is a regular tetrahedron, i.e. its sides are of equal length. E and F are the mid-points of AB and CD respectively. Find the angle between a) the plane ABEF and the plane CDEF b) the plane ABEF and the line AC c)the line AC and the line EF 2. Relevant equations 3. The attempt at a solution For (a), does plane ABEF same as plane ABF while plane CDEF same as CDE? Can anyone give me some hints? Thank you.