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


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[1] := Remove["Global`*"]

In[2] := myEqn1 = -Dt[u,x] == r i + l Dt[i,t]

In[3] := myEqn2 = -Dt[i,x] == s u + c Dt[u,t]



Direct approach: Using "Solve"

In[4] := Solve[{myEqn1,myEqn2}, {Dt[u,{x,2}]}]

results an empty set of solutions:

{{}}



Second try: Using "Reduce"

In[5] := 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[6] := 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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