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<p>tetrahedral faces OAB, OAC, OBC and ABC.</p>

<p>A useful formula for calculating the solid angle of the tetrahedron at the origin O that is purely a function of the vertex angles , ,  is given by <a href="page.php?w=L%27Huilier%27s_theorem">L'Huilier's theorem</a> as</p>

<p>where</p>

<p>Another interesting formula involves expressing the vertices as vectors in 3 dimensional space. Let  be the vector positions of the vertices A, B and C, and let , , and  be the magnitude of each vector (the origin-point distance). The solid angle  subtended</p><p>
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