Re: Problem with Mathematica driving me nuts
- To: mathgroup at smc.vnet.net
- Subject: [mg46824] Re: Problem with Mathematica driving me nuts
- From: "David W. Cantrell" <DWCantrell at sigmaxi.org>
- Date: Tue, 9 Mar 2004 04:31:14 -0500 (EST)
- References: <c2he1m$ahd$1@smc.vnet.net>
- Sender: owner-wri-mathgroup at wolfram.com
bpw67deletethis at hotmail.com (benwoodward.com) wrote: > FindRoot[x^2 == 4x - 4, {x, 1}] > > Out[4]= > {x -> 1.99902} > > In[15]:= > FindRoot[x^2 - 4*x + 4, {x, 3}] > > Out[15]= > {x -> 2.00098} > > When the root is clearly two. > Is Mathematica using Newton's Method like a Ti-92? > Even if so, why wont it give a more accurate answer? > I've tried N[%,30] but it doesn't do anything. > I'm new to Mathematica coming over from a Ti-92, so everything is > frustrating right now. I'm surprised the answers you got were not more accurate. [I suppose you're familiar with the fact that Solve and Reduce can be used to get the precise answer.] Using version 5.0.0: In[1]:= FindRoot[x^2 == 4x - 4, {x, 1}] Out[1]= {x -> 2.} In[2]:= FindRoot[x^2 - 4x + 4, {x, 3}] Out[2]= {x -> 2.} The above outputs certainly _look_ nice. But FindRoot is a numerical method after all, and so the values given are not precisely 2 : In[3]:= FullForm[{%%,%}] Out[3]//FullForm= List[List[Rule[x,1.9999999701976776`]],List[Rule[x,2.0000000298023224`]]] HTH, David