The Math Forum



Search All of the Math Forum:

Views expressed in these public forums are not endorsed by NCTM or The Math Forum.


Math Forum » Discussions » sci.math.* » sci.math

Topic: Help on tetrahedrons
Replies: 1   Last Post: Jun 16, 1996 6:03 PM

Advanced Search

Back to Topic List Back to Topic List Jump to Tree View Jump to Tree View  
hawthorn@waikato.ac.nz

Posts: 14
Registered: 12/12/04
Re: Help on tetrahedrons
Posted: Jun 16, 1996 6:03 PM
  Click to see the message monospaced in plain text Plain Text   Click to reply to this topic Reply



In article <4poknv$9jo@rzsun02.rrz.uni-hamburg.de>, fc3a501@GEOMAT.math.uni-hamburg.de (Hauke Reddmann) writes:
> Maciej Kosinski (atomko1@ato.abb.se) wrote:
> : How to prove that the angule of the regular tetrahedron is 109
> : degrees (or was it 109,5 degrees) in NH3 molecule?
>
> Old trick: Use CH4 instead. All coordinates must sum to zero
> by symmetry, so if one H is at (1,0,0), the rest is at (-1/3,?,??).
> You make take ? as 0. Apply vector cosinus. cos(angle)=-1/3.Done.


A better idea is to note that the points with co-ords
(1,1,1) (1,-1,-1) (-1,1,-1) (-1,-1,1)
are the vertices of a tetrahedron centered at the origin.

Use the dot product to obtain cos(t) = -1/3

Ian H







Point your RSS reader here for a feed of the latest messages in this topic.

[Privacy Policy] [Terms of Use]

© The Math Forum at NCTM 1994-2017. All Rights Reserved.