Re: Why is this so?
- To: mathgroup at smc.vnet.net
- Subject: [mg15311] Re: [mg15304] Why is this so?
- From: "Kevin J. McCann" <kevinmccann at Home.com>
- Date: Fri, 8 Jan 1999 04:15:04 -0500
- Sender: owner-wri-mathgroup at wolfram.com
Yes, but that is not the only possible answer. Try N[(-125)^(1/3)] and you get 2.5+4.33013 I which is why Plot gets upset. E.g. NRoots[x^3 == -125,x] gives x==-5.||x==2.5 - 4.33013 I||x==2.5 +4.33013 I Mathematica picks the one it does, I suspect, because it is the first one you come to in the complex plane if you start at the x-axis (phi=0) and crank around CCW. Kevin which is one of the 3 roots. -----Original Message----- From: Chester Lin <chester at nicco.sscnet.ucla.edu> To: mathgroup at smc.vnet.net Subject: [mg15311] [mg15304] Why is this so? >The following in/out does not make sense to me: > >Clear[f, x] >f[x_] := x^(1/3) >Plot[f[x], {x, -125, 125}] > >Plot::plnr : f[x] is not a machine-size real number at x = -125.. >Plot::plnr : f[x] is not a machine-size real number at x = -114.858. >Plot::plnr : f[x] is not a machine-size real number at x = -103.798. >General::stop : > Further output of Plot::plnr will be suppressed during this >calculation. > >Isn't it true that (-125)^(1/3) == -5? > >Why do I get this strange result? > >I am using Mathematica 3.01 for Students on Macintosh. > >Thanks for any info. > >Chester Lin >chester at nicco.sscnet.ucla.edu > > >