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MathGroup Archive 2006

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Re: General--Differential equation problem..

  • To: mathgroup at smc.vnet.net
  • Subject: [mg66295] Re: General--Differential equation problem..
  • From: "Stratocaster" <stotz1 at verizon.net>
  • Date: Mon, 8 May 2006 00:46:27 -0400 (EDT)
  • References: <e3jttb$94n$1@smc.vnet.net>
  • Sender: owner-wri-mathgroup at wolfram.com

<randomtalk at gmail.com> wrote in message news:e3jttb$94n$1 at smc.vnet.net...
> hello, i was trying to solve a set of differential equations needed for a 
> school project.. However, i'm completely new to mathematica 5.0, and i 
> have downloaded trial, here is what i inputed into the software:
>
> In[2]:=
> DSolve[{x'[s] == Cos[z], y'[s] == Sin[z], z'[s] == 1/r, r == 2}, {x, y, 
> z}, s]

I'm not sure how new you are to Mathematica, so please don't take offense if 
this is common knowledge to you.  You didn't type the In[2]:= Right?  If you 
did then don't, if you didn't then great (sorry to have doubted you)!

> returns this error: DSolve::dvnoarg: The function z appears with no 
> arguments.

And it is doing this because you have declared z a function within the 
DSolve statement.
DSolve[{These Differential Equations},{for these functions of s}, with 
independent variable s].

> Is there anything wrong with what i inputed?

Yes.  The function z appears with no arguments.  Try using this:
DSolve[{x'[s] == Cos[z[s]], y'[s] == Sin[z[s]], z'[s] == 1/2}, {x[s], y[s],
z[s]}, s]

You see Cos[z[s]]?  z makes no sense if you are solving for z[s].
Notice also that for the second argument I used {x[s],y[s],z[s] }?
I also removed the "r == 2" and just plugged "2" for the equation regarding 
z[s].

Did this give you what you needed? 


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