We've got a tetrahedron ABCD, in which the BCA = BAD,
a sphere with center S appended to this tetrahedron is tangent to the wall
ABC in the middle of the circle described on this wall. To prove that a straight lines
AD and AS are perpendicular.
(Note: The sphere appended to the tetrahedron is the sphere tangent to exactly
one wall and the other three planes containing the wall.)