Why is MATLAB "solve" not finding a solution?

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Kevin Bachovchin
Kevin Bachovchin 2013-12-9
评论： Walter Roberson 2013-12-9
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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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
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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