Create a tetrahedron starting with a square ABCD (clockwise), add 2 points E and F placing them halfway between AB and AD respectively.
Fold along lines EF, EC and FC.
Then, form the tetrahedron by keeping triangle EFC as the base, and folding triangles AEF, BEC, and DFC up so that points A, B, and D all meet.
For AB=a find the radius of the largest sphere contained by this tetrahedron
For a tetrahedron we have two formulas for the volume
Volume = (Area of Base) x Height / 3 and
Volume = (Surface Area) x Inradius / 3. Hence
Inradius = (Area of Base) x Height / (Surface Area).
Initially take AB=1 and use AEF as base. Then Area of Base = 1/8,
Height = 1, and Surface Area = 1 so that Inradius = 1/8. Multiplying up
by a then gives the general result
Inradius = a/8.

Posted by Richard
on 20050420 06:23:09 