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Re: Cross results?
*To*: mathgroup at smc.vnet.net
*Subject*: [mg63205] Re: [mg63201] Cross results?
*From*: Andrzej Kozlowski <akoz at mimuw.edu.pl>
*Date*: Mon, 19 Dec 2005 07:00:58 -0500 (EST)
*References*: <200512181234.HAA24522@smc.vnet.net>
*Sender*: owner-wri-mathgroup at wolfram.com
On 18 Dec 2005, at 21:34, Virgil Stokes wrote:
> 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?
It would never make sense to set 0.0 * u to 0.0, where u is a
symbol since there is nothing known about the size of u. In general,
u could be arbitrarily large and in fact it can even be Infinity. Of
course for real x the value of 0. Cos[x]*Sin[x] will be approximately
0, but by taking x to be a purely imaginary number of huge modulus we
can make Cos[x]*Sin[x] also have arbitrarily large modulus. In fact,
taking x= I Infinity we get:
0. Sin[x]*Cos[x]/.x->I*Infinity
Indeterminate expression DirectedInfinity, encountered.
Indeterminate
In any case, as a rule Mathematica never replaces 0.* non numeric
symbolic expression by 0. With numeric symbols the situation is of
course quite different (for the obvious reasons):
0.*Pi
0.
Andrzej Kozlowski
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