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MathGroup Archive 2003

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RE: Log Equation

  • To: mathgroup at
  • Subject: [mg44749] RE: [mg44744] Log Equation
  • From: "Florian Jaccard" <florian.jaccard at>
  • Date: Tue, 25 Nov 2003 00:45:17 -0500 (EST)
  • Reply-to: <florian.jaccard at>
  • Sender: owner-wri-mathgroup at

Yes, the équation you want to solve is transcendental, so the only ways to
solve it are numerical.

The most simple way is to use FindRoot.
But first, don't forget to draw f(x)=(1-x)/Log(x)-y ! You have to see an
approximation of the root you are searching !

For example, let y = -2  :

In[1]:= Plot[(1 - x)/Log[x] + 2, {x, 0, 10},PlotRange -> {-4, 4}];

In[2]:= FindRoot[(1 - x)/Log[x] == -2, {x, 2}]

Meilleures salutations

Florian Jaccard

-----Message d'origine-----
De : Bernard Bourée [mailto:bernard at]
Envoyé : lun., 24. novembre 2003 06:05
À : mathgroup at
Objet : [mg44744] Log Equation

I 'm trying to find a way to solve the equation

y = (1-x)/ Log(x)

How can I find x when y is known ?

Is there a numerical method ?
with series development?

Bernard Bourée
bernard at

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