Empty sym: 0-by-1 : Don't understand why

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Benoît
Benoît 2024-11-11，15:43
评论： Benoît 2024-11-11，19:29
Hi,
I have the following problem :
I would like to solve this equation system but matlab doesn't find any solutions by sending the following message : Empty sym: 0-by-1
Do i need to adapt the code in a certain way or this system really doesn't have any solutions ?
Thank you for your help.
Regards,
Benoit
  2 个评论
Bruno Luong
Bruno Luong 2024-11-11，16:09
编辑：Bruno Luong 2024-11-11，18:31
Why there is the comma?
3,981
Post your code (using Insert a Line of Code button) not the screen shot
Walter Roberson
Walter Roberson 2024-11-11，18:15
The comma is causing the problem. You are effectively creating four equations,
3
981==EXPRESSION
10*pi==EXPRESSION
6283185.307==EXPRESSION
The first of those expressions, 3, cannot be solved to be equal to zero, so the solve() fails.

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采纳的回答

Torsten
Torsten 2024-11-11，18:56
编辑：Torsten 2024-11-11，19:20
Ran in:
I multiplied by the denominators and squared to get rid of the square roots.
Your system seems to have 4 (complex-valued) solutions.
syms Cs C2 Rl
R1 = 1000;
gm = 10;
eqns = [3.981*(gm*(2*Rl*C2+R1*Cs)+Cs) == gm*Rl*Cs*(1-gm*R1),...
(10*pi*(gm*(2*Rl*C2+R1*Cs)+Cs))^2==2*gm*Rl*C2*Cs*(gm*R1+1),...
6283185.307^2*(Rl*C2*Cs*(1+R1*gm))==2*gm];
S = solve(eqns,[Cs C2 Rl]);
format long
double(S.Cs)
ans =
1.0e-09 * 0.005065552626999 + 0.318237745447037i -0.005065552626999 - 0.318237745447037i 0.005065552626999 - 0.318237745447037i -0.005065552626999 + 0.318237745447037i
double(S.C2)
ans = 4×1
1.0e-04 * -0.125571466971436 0.125571466971436 -0.125571466971436 0.125571466971436
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double(S.Rl)
ans =
-0.000201720156037 + 0.012672845866091i -0.000201720156037 + 0.012672845866091i -0.000201720156037 - 0.012672845866091i -0.000201720156037 - 0.012672845866091i
eqns_orig = [3.981 == gm*Rl*Cs*(1-gm*R1)/(gm*(2*Rl*C2+R1*Cs)+Cs),...
10*pi==(2*gm*Rl*C2*Cs*(gm*R1+1))^(1/2)/(gm*(2*Rl*C2+R1*Cs)+Cs),...
6283185.307==(2*gm/(Rl*C2*Cs*(1+R1*gm)))^(1/2)];
for i = 1:4
double(subs(eqns_orig,[Cs C2 Rl],[S.Cs(i) S.C2(i) S.Rl(i)]))
end
ans = 1×3
0 0 0
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ans = 1×3
0 0 0
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ans = 1×3
0 0 0
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ans = 1×3
0 0 0
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  1 个评论
Benoît
Benoît 2024-11-11，19:29
Thank for your answer.
It is now solving the system but i shouldn't be obtaining complex solutions so i think there is a problem in my equations.
But thanks for the correction.

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