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

**Follow-Ups**:**Re: Re: Cross results?***From:*Andrzej Kozlowski <akoz@mimuw.edu.pl>