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Re: plot command

  • To: mathgroup at
  • Subject: [mg58126] Re: plot command
  • From: Bill Rowe <readnewsciv at>
  • Date: Sun, 19 Jun 2005 03:43:29 -0400 (EDT)
  • Sender: owner-wri-mathgroup at

On 6/18/05 at 6:07 AM, bosch at (Bosch, Darrell) wrote:

>Dear Group:  I am trying to solve an equation, take the derivative
>and plot the derivative as shown below.  I get a message below the
>Plot request stating

>"Syntax::sntxi: "Incomplete expression; more input is needed."  Any
>help would be greatly appreciated.  Darrell Bosch

>sol = Log[v]=A9100*Log(s)

The line immediately above should have generated an error message. It asks Mathematica to set Log[v] to A9100*Log(s). Note, in Mathematica, the expression A9100*Log(s) isn't valid syntax. It would be parsed as the product of three variables A9100, Log and s. And since Log is already a reserved named, this is going to be a problem. Correct syntax would be:

sol = Log[v]==A91000*Log[s]

>sol2 = Solve[sol,v]

Making the change I indicated, sol2 should become

{{v -> s^A9100}}

a list of replacement rules, So,

>sol3 = D[sol2,s]

Needs to become

sol3 = D[v/.First@sol2,s]

which sets sol3 to

A9100*s^(A9100 - 1)

But, there is still one more thing to be done before this can be plotted. That is to assign a value to A9100. So, a plot could be generated as:

Plot[sol3 /. A9100 -> 1/2, {s, 0, 100}]; 

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