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Re: FindRoot with Logarithmic terms

  • To: mathgroup at smc.vnet.net
  • Subject: [mg116029] Re: FindRoot with Logarithmic terms
  • From: Bob Hanlon <hanlonr at cox.net>
  • Date: Sat, 29 Jan 2011 05:29:34 -0500 (EST)

expr = 7500*x*(1 - x) + 325*8.314*(x*Log[x] + (1 - x)*Log[(1 - x)]);

expr /. {{x -> 0}, {x -> 1}} // Quiet

{Indeterminate, Indeterminate}

So give it different starting values than 0 and 1

FindRoot[expr == 0, {x, 0.1, 0.2}] // Chop

{x -> 0.468458}

FindRoot[expr == 0, {x, 0.8, 0.9}] // Chop

{x -> 0.531542}


Bob Hanlon

---- skunkwerk <skunkwerk at gmail.com> wrote: 

=============
Hi,
  I'm trying to find the roots of a logarithmic equation that I've plotted successfully (I can see both roots are between 0 and 1).

expr = 7500*x*(1 - x) + 325*8.314*(x*Log[x] + (1 - x)*Log[(1 - x)])
FindRoot[expr == 0, {x, 0, 1}]

FindRoot:nlnum: the function value(indeterminate) is not a list of numbers with dimensions {1} at {x} = {0.}

any ideas?

thanks



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