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Tetrahedron bounded by spheres (Posted on 2003-04-17) Difficulty: 3 of 5
A regular tetrahedron holds a sphere snugly within its four sides. A larger sphere surrounds the tetrahedron, just touching its four vertices. What is the ratio of radii of the two spheres?

  Submitted by Bryan    
Rating: 3.5000 (6 votes)
Solution: (Hide)
Start by considering the center of the spheres, which is also the geometric center of the tetrahedron, which we will call T. From this center, draw a ray to each vertex of tetrahedron T. These rays form the edges of four smaller, irregular tetrahedrons, t1 thru t4, each with the center point of T at its apex and a side of T as its base. From symmetry, t1 thru t4 are dimensionally identical.

In general, the volume of a tetrahedron is 1/3*base*height. Since t1 is 1/4 the volume of T (t1 thru t4 add up to T)and has the same base, the height of t1 must be 1/4 the height of T. And since the height of t1 plus the length of a ray equal the height of T, the length of each ray must be 3/4 the height of T.

Since the height of t1 is the radius of the inner sphere, and the ray is the radius of the outer sphere, the ratio of radii for the two spheres is 1/4 to 3/4, or 1:3.

Comments: ( You must be logged in to post comments.)
  Subject Author Date
answerK Sengupta2007-07-30 13:30:40
SolutionSolutionAntonio2003-08-21 19:09:41
re: Solution (long and complicated version)Charlie2003-04-17 10:04:54
SolutionThis is the solutionCharlie2003-04-17 09:42:15
I getjude2003-04-17 09:41:56
SolutionSolution (short version)Hank2003-04-17 09:31:11
Solution (long and complicated version)Hank2003-04-17 09:30:41
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