Re: General--Differential equation problem..

*To*: mathgroup at smc.vnet.net*Subject*: [mg66303] Re: General--Differential equation problem..*From*: "Scout" <Scout at nodomain.com>*Date*: Mon, 8 May 2006 00:46:56 -0400 (EDT)*References*: <e3jttb$94n$1@smc.vnet.net>*Sender*: owner-wri-mathgroup at wolfram.com

<randomtalk at gmail.com> 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] > > returns this error: DSolve::dvnoarg: The function z appears with no > arguments. > > Is there anything wrong with what i inputed? The differential equations > came from this website: http://fy.chalmers.se/LISEBERG/eng/loop_pe.html > > Link to the forum page for this post: > http://www.mathematica-users.org/webMathematica/wiki/wiki.jsp?pageName=Special:Forum_ViewTopic&pid=10212#p10212 > Posted through http://www.mathematica-users.org [[postId=10212]] > > Hi, in your set of coupled differential equations, you have forgotten to add the independent variable s to the function z. The r == 2 equation causes an error inside DSolve[], because it's not a differential equation. Try this: In[1]:= diff=DSolve[{x'[s] == Cos[z[s]], y'[s] == Sin[z[s]], z'[s] == 1/r}, {x[s], y[s], z[s]}, s] Out[1]= {{z[s] -> s/r + C[1], x[s] -> C[2] + r*Cos[C[1]]*Sin[s/r] + r*Cos[s/r]*Sin[C[1]], y[s] -> C[3] - r*Cos[s/r]*Cos[C[1]] + r*Sin[s/r]*Sin[C[1]] }} In[2]:= diff /. r->2 Out[2]= {{z[s] -> s/2 + C[1], x[s] -> C[2] + 2*Cos[C[1]]*Sin[s/2] + 2*Cos[s/2]*Sin[C[1]], y[s] -> C[3] - 2*Cos[s/2]*Cos[C[1]] + 2*Sin[s/2]*Sin[C[1]] }} Regards, ~Scout~