ABCD is a regular tetrahedron (right pyramid whose faces are all equilateral triangles). If M is the midpoint of line segment CD, then what is cos∠ABM?
Please help me, thanks.
ABCD is a regular tetrahedron (right pyramid whose faces are all equilateral triangles).
If M is the midpoint of line segment CD , then what is cos
See if you can make sense of my pic and then use cosine rule.
+1633 
