MATLAb Solve function not solving

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Antonio Olinto Tonisi
Antonio Olinto Tonisi 2024-10-21，18:34
回答： Steven Lord 2024-10-21，18:54
I am trying to solve a system of equations in Matlab, however when I run the code below, it doesn't return an answer for I1, I2 and V0, although there 3 equations and 3 variables. I need some help.
clear all
close all
syms I1 I2 V0
s=2;
eqns=[5*I1+4*I2+1/(s*0.1)*I1+2*s*I1==1, 4*I2-3*V0==0, V0==I1*(2*s)];
S=solve(eqns,[I1,I2,V0])

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采纳的回答

Steven Lord
Steven Lord 2024-10-21，18:54
I ran the code using release R2019a and it seems to have worked correctly, giving results that match what others have posted in previous answers.
>> syms I1 I2 V0
s=2;
eqns=[5*I1+4*I2+1/(s*0.1)*I1+2*s*I1==1, 4*I2-3*V0==0, V0==I1*(2*s)];
S=solve(eqns,[I1,I2,V0])
S =
struct with fields:
I1: [1×1 sym]
I2: [1×1 sym]
V0: [1×1 sym]
>> S.I1
ans =
1/26
>> S.I2
ans =
3/26
>> S.V0
ans =
2/13
>> version
ans =
'9.6.0.1072779 (R2019a)'
The display of the struct S doesn't show the values of the symbolic expressions but the fields do contain those values as you can see by displaying each of the individual fields on their own.
Can we check that those values are the correct solutions? Sure, substitute the results back into the equations using subs.
>> subs(eqns, S)
ans =
[ 1 == 1, 0 == 0, 2/13 == 2/13]
Those look like true expressions to me.

更多回答（3 个）

Torsten
Torsten 2024-10-21，18:38
Ran in:
Which MATLAB version are you using ? R2024b gives a direct answer:
clear all
close all
syms I1 I2 V0
s=2;
eqns=[5*I1+4*I2+1/(s*0.1)*I1+2*s*I1==1, 4*I2-3*V0==0, V0==I1*(2*s)];
S=solve(eqns,[I1,I2,V0])
S = struct with fields:
I1: 1/26 I2: 3/26 V0: 2/13
Star Strider
Star Strider 2024-10-21，18:42
编辑：Star Strider 2024-10-21，18:47
Ran in:
Your code works in R2024b —
clear all
close all
syms I1 I2 V0
s=2;
eqns=[5*I1+4*I2+1/(s*0.1)*I1+2*s*I1==1, 4*I2-3*V0==0, V0==I1*(2*s)];
S=solve(eqns,[I1,I2,V0])
S = struct with fields:
I1: 1/26 I2: 3/26 V0: 2/13
format long
I1 = double(S.I1)
I1 =
0.038461538461538
I2 = double(S.I2)
I2 =
0.115384615384615
V0 = double(S.V0)
V0 =
0.153846153846154
Consider upgrading, if that is an option. (I no longer have access to R2019a to see if there could be a work-around.)
EDIT — Changed vpa calls to double, since symbolic results no longer appear here after a post is submittted.
.
Taylor
Taylor 2024-10-21，18:45
Ran in:
Seems to work fine here
clear all
close all
syms I1 I2 V0
s=2;
eqns=[5*I1+4*I2+1/(s*0.1)*I1+2*s*I1==1, 4*I2-3*V0==0, V0==I1*(2*s)];
S=solve(eqns,[I1,I2,V0])
S = struct with fields:
I1: 1/26 I2: 3/26 V0: 2/13

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