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MathGroup Archive 2004

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Re: Re: Non-algebraic solution

  • To: mathgroup at smc.vnet.net
  • Subject: [mg52458] Re: [mg52421] Re: [mg52399] Non-algebraic solution
  • From: DrBob <drbob at bigfoot.com>
  • Date: Sun, 28 Nov 2004 01:06:38 -0500 (EST)
  • References: <Pine.LNX.4.44.0411271159360.16427-100000@crest.ernet.in>
  • Reply-to: drbob at bigfoot.com
  • Sender: owner-wri-mathgroup at wolfram.com

In that case, symbolic solutions are probably impossible.

But numerical solutions are still doable:

Clear[ySolve]
ySolve[a_, b_, c_, d_][(x_)?NumericQ] :=
   E^logY /. First[FindRoot[
      Log[E^logY/a] + b*Log[c + E^logY/d] == -x, {logY, 50.}]]
plot = Plot[ySolve[3.1, 2.5, 1.07, 4.3][x], {x, -25, 5}];

Bobby

On Sat, 27 Nov 2004 12:01:35 +0530 (IST), B.Ravinder <ravi at crest.ernet.in> wrote:

> But the problem in my case is that a, b, c, d are Non-integer number.
>
>
>
> On Fri, 26 Nov 2004, DrBob wrote:
>
>> It can be solved and plotted for some values of the parameters.
>>
>> For instance:
>>
>> Clear[x, y]
>> Block[{a = 3,
>>   b = 2, c = 1, d = 4, equation = Log[y/a] + b*Log[c + y/d] == -x},
>>      Solve[equation, y]
>>      ];
>> y[x_] = y /. First@%
>> Plot[y@x, {x, -25, 10}]
>>
>> Bobby
>>
>> On Fri, 26 Nov 2004 01:04:27 -0500 (EST), B.Ravinder <ravi at crest.ernet.in> wrote:
>>
>> > Dear all,
>> > I want to plot the following type of logarithmic equation having
>> > dependent variable y and independent variable x.
>> >
>> > Log[y/a] + b*Log[c+y/d] = -x
>> >
>> > where a,b,c and d are some arbitrary constants.
>> >
>> > First I tried to solve the equation for y using Solve function in
>> > Mathematica 4.2, but got the following error
>> > message:
>> > Solve:: tdep :
>> > The equations appear to involve the variables to be solved
>> > for in an essentially non-algebraic way.
>> >
>> >
>> > Could someone please guide me as to how do we solve such equation
>> > with/without Mathematica.
>> >                              or
>> > If possible, how to obtain y vs. x plot in Mathematica without soving them
>> > exactly.
>> > Waiting for the kind resonse.
>> >
>> > Regards,
>> > Ravi
>> >
>> >
>> >
>> >
>>
>>
>>
>>
>
>
>
>



-- 
DrBob at bigfoot.com
www.eclecticdreams.net


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