$ABCD$ is a regular tetrahedron (right triangular pyramid).  If $M$ is the midpoint of $\overline{CD}$, then what is $\cos \angle AMB$?
Question:
$ABCD$ is a regular tetrahedron (right triangular pyramid).  If $M$ is the midpoint of $\overline{CD}$, then what is $\cos \angle AMB$?
Answer:
Let's think step by step.

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$ABCD$ is a regular tetrahedron (right triangular pyramid).  If $M$ is the midpoint of $\overline{CD}$, then what is $\cos \angle AMB$?
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