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MathGroup Archive 1998

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Re: new user help

  • To: mathgroup at smc.vnet.net
  • Subject: [mg13328] Re: new user help
  • From: Arnoud Buzing <arnoudb>
  • Date: Mon, 20 Jul 1998 02:49:50 -0400
  • Organization: Wolfram Research, Inc.
  • References: <6okkvj$1md@smc.vnet.net>
  • Sender: owner-wri-mathgroup at wolfram.com

John M. Dlugosz wrote:
> 
> I'm lost.  I just don't know how to get started...
> 
> The excersize I've chosen for myself is to start with
> 
>         x^2+y^2+4x-6y+4==0
> 
> and manipulate it into the form (x-h)^2+(y-k)^2==r^2
> 
> So... how do I "manipulate" the equasion?  The functions like Expand,
> Factor, etc. don't help much.  I my calculator (an HP48) I can point to
> specific subexpressions and apply operations to them, like factor,
> distribute, changing forms, etc.
> 
> How do I collect the x's together in parens, the y's in parens, and
> complete the squares?  Doing it on paper defeats the point!  I want to
> learn how to "do math" using this tool.  That's more than just asking
> "OK, what's X?".  It means manipulating things and arranging them,
> getting to know how the symbols all fit together.
> 
> --John

This may help: 

x^2 + y^2 + 4x - 6y + 4 == 0 //. {t_^2 + b_ t_ -> (t + b/2)^2 - b^2/4} 

The //. is short for ReplaceRepeated, which means it will apply the rule
on the right until there is nothing left to replace.

The { ... -> ... } is a replacement rule. In this case it will look for
things that look like t^2+bt and replace it by what you suggested. (See
also section 2.4 in the Mathematica book).

---
Arnoud


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