       Simplifying a second order eq. system tests=PRIORITY_NO_NAME version=2.55

• To: mathgroup at smc.vnet.net
• Subject: [mg45928] Simplifying a second order eq. system tests=PRIORITY_NO_NAME version=2.55
• From: "Gunnar Lindenblatt" <Gunnar.Lindenblatt at pobox.com>
• Date: Fri, 30 Jan 2004 04:15:54 -0500 (EST)
• Sender: owner-wri-mathgroup at wolfram.com

```Hi,

I just switched from another system to Mathematica
5. While all "complicated" operations (like importing Wave-Files, filtering,
parameter fitting, plotting) just work fine, I still have problems with the
"basics" like solving or even simplifying equations:

For example, to get the telegraph equation by self-induction and capacitive
coupling:

(One can solve this problem on the space of a postage stamp...)

In := Remove["Global`*"]

In := myEqn1 = -Dt[u,x] == r i + l Dt[i,t]

In := myEqn2 = -Dt[i,x] == s u + c Dt[u,t]

Direct approach: Using "Solve"

In := Solve[{myEqn1,myEqn2}, {Dt[u,{x,2}]}]

results an empty set of solutions:

{{}}

Second try: Using "Reduce"

In := Reduce[{myEqn1,myEqn2}, {Dt[u,{x,2}]}]

results:

Reduce::nsmet: This system cannot be solved with the methods available to
Reduce.

Third try: Using "Eliminate"

In := Eliminate[{myEqn1,myEqn2}, {Dt[i,x],Dt[i,t]}]

results:

True

That's fine! However, it does not really help me...

(By the way, the result should be:

Dt[u,{x,2}]== r s u + (r c + l s) Dt[u,t] + l c Dt[u,{t,2}])

Any ideas? -- Perhaps this problem is too simple for Mathematica, so it
rejects any help ;-)

- Gunnar

--
Gunnar Lindenblatt
e-mail: Gunnar.Lindenblatt at pobox.com

```

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