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Re: Cross results?


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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