All about flooble | fun stuff | Get a free chatterbox | Free JavaScript | Avatars    
perplexus dot info

Home > Probability
Four Point Spheres (Posted on 2012-01-22) Difficulty: 3 of 5
Four points are chosen at random on the surface of a sphere. Determine the probability that the center of the sphere lies inside the tetrahedron described by these four points.

No Solution Yet Submitted by K Sengupta    
No Rating

Comments: ( Back to comment list | You must be logged in to post comments.)
Some Thoughts I think he's got it. Comment 2 of 2 |
I agree with Dustin.

If the points are all in a single hemisphere, then they clearly do not contain the center (except in the zero probability case where three of the points lie on a great circle).

Working in the other direction, it is also pretty clear that if the tetrahedron does not contain the center, then there must be planes which divide the sphere in half and which do not intersect the tetrahedron.  Therefore, if the tetrahedron does not contain the sphere's center, then its points all lie in a single hemisphere. Contrapositively, if all of its points are not in a single hemisphere, then the tetrahedron does contain the sphere's center.

Therefore,  the center of the sphere lies inside the tetrahedron described by these four points if and only if the four points do not lie in a single hemisphere.   



  Posted by Steve Herman on 2012-01-23 11:38:18
Please log in:
Login:
Password:
Remember me:
Sign up! | Forgot password


Search:
Search body:
Forums (0)
Newest Problems
Random Problem
FAQ | About This Site
Site Statistics
New Comments (8)
Unsolved Problems
Top Rated Problems
This month's top
Most Commented On

Chatterbox:
Copyright © 2002 - 2024 by Animus Pactum Consulting. All rights reserved. Privacy Information