We've got a tetrahedron ABCD, in which the http://matholympiad.org.bd/forum/jsM...100/char5C.pngBCA = http://matholympiad.org.bd/forum/jsM...100/char5C.pngBAD,
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.)
Google Tetrahedron Wiki and then edit your post
Sorry for any mistakes but I translated it from my mother language and my english is not perfect