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MathGroup Archive 2005

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Re: Cross results?

  • To: mathgroup at smc.vnet.net
  • Subject: [mg63206] Re: [mg63201] Cross results?
  • From: "David Park" <djmp at earthlink.net>
  • Date: Mon, 19 Dec 2005 07:00:59 -0500 (EST)
  • Sender: owner-wri-mathgroup at wolfram.com

Virgil,

Because Mathematica is doing approximate number arithmatic it isn't certain
that the result is exactly zero - although in this case it is.

Either use Chop or exact calculations.

Cross[{10 Sin[x], -10 Cos[x], 2.5}, {0.12 Sin[x], -0.12 Cos[x], 0.0}] //
Chop
{0.3 Cos[x], 0.3 Sin[x], 0}

Cross[{10 Sin[x], -10 Cos[x], 5/2}, {12/100 Sin[x], -12/100 Cos[x], 0}]
{(3*Cos[x])/10, (3*Sin[x])/10, 0}

David Park
djmp at earthlink.net
http://home.earthlink.net/~djmp/




From: Virgil Stokes [mailto:virgil.stokes at it.uu.se]
To: mathgroup at smc.vnet.net


I have the following

  x=.
  Cross[{10 Sin[x],-10 Cos[x],2.5},{0.12 Sin[x],-0.12 Cos[x], 0.0}]

which gives,

  {0.3 Cos[x], 0.3 Sin[x], 0.Cos[x]Sin[x]}

why does it not set the last term to 0.0?



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