Why is MATLAB "solve" not finding a solution?

2013 12 9
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更新时间：2013 12 9
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Hello,
I have a system of equations:
-id = 0
qd/(C*L) - (Vs - R*id)/L = 0
I'm trying to solve this system of equations for id and Vs using the MATLAB solve function. After defining all variables as syms, I'm using the below code
Equations = [-id ; qd/(C*L) - (Vs - R*id)/L]
Solution = solve(Equations==0,id,Vs)
However, this is producing a warning, "Explicit solution could not be found." Why isn't MATLAB able to find a solution here?
There is a solution (id = 0, Vs = qd/C) which can be easily solved by hand.
Thank you,
Kevin

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Jie
Jie 2013-12-9
Not all equations have explicit solutions. Try numerical solution by fsolve or fzero. To be honest, I don't understand the way you write your equation much. I wish I could help..
Kevin Bachovchin
Kevin Bachovchin 2013-12-9
I'm aware that not all system of equations have explicit solutions, but it seems like this one does (and can be easily solved by hand).
What specifically don't you understand about my equations? My system of two equations and two unknowns is:
-id = 0
qd/(C*L) - (Vs - R*id)/L = 0
The other variables besides id and Vs are known parameters.
Kevin Bachovchin
Kevin Bachovchin 2013-12-9
Also I'm not looking for a numerical solution, I'm looking for a symbolic solution. I've used the "solve" command successfully before. I'm not sure why it is failing on this system.
Kevin Bachovchin
Kevin Bachovchin 2013-12-9
I've been trying to debug this problem and I found that
solve(-xd == 0, xd) yields xd = 0 like it is supposed to
while solve(-id == 0, id) yields no solution can be found. I don't understand what is causing the difference. Can someone please help me?

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Walter Roberson
Walter Roberson 2013-12-9

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Potentially it is a clash with a MuPAD name. See http://www.mathworks.com/help/symbolic/mupad_ref/id.html

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Kevin Bachovchin
Kevin Bachovchin 2013-12-9
Ok, thanks. Any idea how to get around this issue (other than using a new variable name)?
Walter Roberson
Walter Roberson 2013-12-9
Sorry, I do not have that toolbox to test with.

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