[list]
[*] [i]r[/i] = radius of sphere
[*] [i]l[/i] = length of AB
[/list]
[math]
\left| \overrightarrow{PA} \right| ^2 +
\left| \overrightarrow{PB} \right| ^2 +
\left| \overrightarrow{PC} \right| ^2 +
\left| \overrightarrow{PD} \right| ^2
=
8 r^2
=
3 l^2
[/math]
