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MathGroup Archive 1999

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Re: Linear algebra

  • To: mathgroup at smc.vnet.net
  • Subject: [mg15990] Re: [mg15876] Linear algebra
  • From: "Richard Finley" <rfinley at medicine.umsmed.edu>
  • Date: Fri, 19 Feb 1999 03:27:07 -0500
  • Sender: owner-wri-mathgroup at wolfram.com

Roland....here is a way to do it using Mma:
Define the unit direction vectors for the diagonal of a cube in a matrix d

d = {{1/Sqrt[3],1/Sqrt[3],1/Sqrt[3]},{-1/Sqrt[3],1/Sqrt[3],1/Sqrt[3]},{1/
Sqrt[3],-1/Sqrt[3],
1/Sqrt[3]},{1/Sqrt[3],1/Sqrt[3],-1/Sqrt[3]}}

Find the unit direction vector of your line ...eg. if r = {r1,r2,r3} is 
the non-unit direction vector then
n = r/Sqrt[r.r]  will be the unit direction vector.

Then your diagonal angles are given by v = ArcCos[d.n//Simplify]

And finally the answer by Sum[Cos[v[[i]]]^2,{i,1,4}]//Simplify

Which will give you the simple fractional answer (that does not depend on 
n!!).  One doesn't actually need to go through all those computations but 
I did it that way to put it more closely into the form you requested.  
Hope that helps......regards,  RF

>>> Roland Bengtsson <d98rolb at stud.hh.se> 02/17/99 10:33PM >>>

Let V1, V2, V3 and V4 be the angles that a given line create with
the diagonals in space in a cube. How can I calc the sum of:

(cos V1)^2 + (cos V2)^2 + (cos V3)^2 + (cos V4)^2

in Mathematica 3.0 or by hand?

All help appreciated!




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