Show that the p.v. of circumcentre of a tetrahedron \( O A B C \) is (where \( O \) is the ori...

Show that the p.v. of circumcentre of a tetrahedron \( O A B C \) is (where \( O \) is the origin) \( \frac{\vec{a}^{2}(\vec{b} \times \vec{c})+\vec{b}^{2}(\vec{c} \times \vec{a})+\vec{c}^{2}(\vec{a} \times \vec{b})}{2[\vec{a} \vec{b} \vec{c}]} \)