matrix math problems , error in matrix size

Question

tyler hollings 2022-10-22

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评论： tyler hollings 2022-10-22

i have a rectangular matrix of size 18 by 3 and a column of 18 by 1, when I try to solve for the x matrix (3 by 1) it says I have incorrect sized dimensions to do the math, why? mat lab says the rows should agree and it should allow me to divide them using "/"

xx=[x_e; x_a; x_w];
 NU=[lnu_e;lnu_a;lnu_w];
co=xx/NU;

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Answer 1

Paul 2022-10-22

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在 MATLAB Online 中打开

Hard to say without without knowing anything about the data in the question. Based on the wording it should work as follows

A = rand(18,3);  % 18 by 3
b = rand(18,1);  % 18 by 1
x = A\b          % solve for 3 x 1, note use of backslash
x = 3×1
    0.5890
    0.4314
    0.1437
ans = 18×1
   -0.1966
   -0.0326
    0.4505
    0.0437
    0.3522
   -0.0633
   -0.0692
   -0.6059
   -0.3226
   -0.0753

The result does not satisfy the matrix equation because there are more equations than unknowns

norm(A*x - b)

See mldivide, \ for more info on how it treats this case.

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Answer 2

Walter Roberson 2022-10-22

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xx = rand(18,3);
NU = rand(18,1);
try
    xx/NU
    fprintf('/ worked\n');
catch ME
    fprintf('well, / did not work\n');
end
well, / did not work
try
    xx\NU
    fprintf('\\ worked\n');
catch ME
    fprintf('well, \\ did not work\n');
end
ans = 3×1
    0.4033
    0.1974
    0.1226
\ worked