matrix math problems , error in matrix size

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;

Answer 1

[Paul]

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.

Answer 2

[Walter Roberson]

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

[tyler hollings]

[tyler hollings]