MATLAB Answers

Solve system of linear equations ...matrix output is not as expected

29 views (last 30 days)
Mark Sc
Mark Sc on 3 Mar 2021
Commented: Mark Sc on 3 Mar 2021
Hi All,
I am trying to solve system of equations, as attached,
I have written a code, but my final matrix is 9*9 however, it should be 3*3....I understand i have 9 unknowns with 9 equations, but i wish to understand how could i solve it..
clearvars;
clc;
syms A11 A12 A13 A21 A22 A23 A31 A32 A33 real;
eq1 = A11*-0.001179+A12*-6.581+A13*0.008718 == -0.5;
eq2 = A12*-0.001179+A22*-6.581+A23*0.008718 == 0;
eq3 = A31*-0.001179+A32*-6.581+A33*0.008718 == 0;
eq4 = A11*-0.00251+A12*0.05848+A13*-0.039518 == 0;
eq5 = A21*-0.00251+A22*0.05848+A23*-0.039518 == -0.5;
eq6 = A31*-0.00251+A32*0.05848+A33*-0.039518 == 0;
eq7 = A11*-0.0051200+A12*0.2779500+A13*0.0001469 == 0;
eq8 = A21*-0.0051200+A22*0.2779500+A23*0.0001469 == 0;
eq9 = A31*-0.0051200+A32*0.2779500+A33*0.0001469 == -0.5;
[A] = equationsToMatrix([eq1, eq2, eq3, eq4, eq5, eq6, eq7, eq8, eq9], [A11, A12, A13, A21, A22,A23, A31, A32, A33]);

Accepted Answer

Steven Lord
Steven Lord on 3 Mar 2021
A being a 9-by-9 matrix is correct.
syms A11 A12 A13 A21 A22 A23 A31 A32 A33 real;
eq1 = A11*-0.001179+A12*-6.581+A13*0.008718 == -0.5;
eq2 = A12*-0.001179+A22*-6.581+A23*0.008718 == 0;
eq3 = A31*-0.001179+A32*-6.581+A33*0.008718 == 0;
eq4 = A11*-0.00251+A12*0.05848+A13*-0.039518 == 0;
eq5 = A21*-0.00251+A22*0.05848+A23*-0.039518 == -0.5;
eq6 = A31*-0.00251+A32*0.05848+A33*-0.039518 == 0;
eq7 = A11*-0.0051200+A12*0.2779500+A13*0.0001469 == 0;
eq8 = A21*-0.0051200+A22*0.2779500+A23*0.0001469 == 0;
eq9 = A31*-0.0051200+A32*0.2779500+A33*0.0001469 == -0.5;
equations = [eq1, eq2, eq3, eq4, eq5, eq6, eq7, eq8, eq9];
v = [A11, A12, A13, A21, A22,A23, A31, A32, A33];
[A, b] = equationsToMatrix(equations, v);
Let's attempt to recreate the original equations using A and b.
A*v.' == b
ans = 
equations.'
ans = 
Those look like they match to me. Now to solve for v:
v2 = A\b
v2 = 
vpa(A*v2-b, 5)
ans = 
  3 Comments
Mark Sc
Mark Sc on 3 Mar 2021
Thanks I see, but I am just afraid as the answer should has some zeros elements,
but thanks again

Sign in to comment.

More Answers (0)

Community Treasure Hunt

Find the treasures in MATLAB Central and discover how the community can help you!

Start Hunting!