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MathGroup Archive 1996

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Re: HELP! algebraic math problem

  • Subject: [mg2950] Re: HELP! algebraic math problem
  • From: bshoels at rs1.tcs.tulane.edu (Brett Shoelson)
  • Date: 14 Jan 1996 05:32:06 -0600
  • Approved: usenet@wri.com
  • Distribution: local
  • Newsgroups: wri.mathgroup
  • Organization: Tulane University
  • Sender: mj at wri.com

DavidA.Johnson at news.acns.nwu.edu wrote:
: I'm stumped!

: I have a simple problem that I certainly should be able to see the solution and my memory just isn't working.  Is there anyone who can help me solve this.

: I have a general equation of: d = a + bx + c ln(x)

: I want to isolate x, how do I do it?

: In reality I know a, b, c, and d and need to calculate x

: I've been trying to solve this in Mathematica and it just doesn't work because of limitations in their solve routine.  Of course I've been lazy for quite a while and have been using Mma to much to solve problems and my mind has obviously forgotten simple concepts.  


: appologetically, and in advance, THANK'S,

: David A. Johnson
: please email any solution or suggestions.

This works, as long as you have numerical values of a,b,c,d. (Otherwise
it's a much more difficult <possible?> problem:

IN:
ClearAll[d,a,b,c,x]
ans = Roots[d==a + b x + c Log[x]/.{a->2,b->6,c->4,d->5},x]
OUT:
x == (3 - 4*Log[x])/6

IN:
FindRoot[ans,{x,1}]
OUT:
{x -> 0.7194822665641975156}

Note that you can also plug this equation into an HP or TI scientific
calculator (or "equivalent"--although there is no equivalent of an HP)
and it will spit out an answer for x.

Regards,

Brett Shoelson
bshoels at rs6000.tcs.tulane.edu


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