Vectors in Space · Superchallenge problem

Tetrahedron $ABCD$ has edge lengths $AB = 5$, $CD = 6$, $AC = 4$, $BD = 6$, $AD = 5$ and $BC = 7$. Find the cosine of the angle between the vectors $\overrightarrow{AB}$ and $\overrightarrow{CD}$.