How do I found A-B*x which these matrix are 3x3

A=[1231200 -615600 0; -615600 1231200 -615600; 0 -615600 615600]

A =

1231200 -615600 0

-615600 1231200 -615600

0 -615600 615600

B=[62.68 0 0; 0 62.68 0; 0 0 62.68]

B =

62.6800 0 0

0 62.6800 0

0 0 62.6800

And I want to get the A-Bx which x is a uknown variable that I want to know.

Answer 1

[Walter Roberson]

>> x=B\A

x =

          19642.6292278239         -9821.31461391193                         0
         -9821.31461391193          19642.6292278239         -9821.31461391193
                         0         -9821.31461391193          9821.31461391193

>> A-B*x

ans =

     0     0     0
     0     0     0
     0     0     0

[Rommel Brito]

In red is the Matrix A-Bx and in green is the result but i cant get the same result in matlab any idea?

[Walter Roberson]

A = [1231200, -615600, 0; -615600, 1231200, -615600; 0, -615600, 615600];
B = [62.68 0 0; 0 62.68 0; 0 0 62.68];
syms L
vals = solve(det(A - B*L), 'MaxDegree', 3);
real(vpa(vals))       %imaginary component is noise

[Rommel Brito]

Thank you! That worked!

Answer 2

[Bruno Luong]

Waoh, revise the theory of eigen vectors and linear algebra in general

Multiply a vector by a constant diagonal matrix is like multiply by the scalar.

This equation admits a solution for only when B = lambda*Identity where lambda is eigen value and x is the corresponding eigen eigen vector.

I'm surprises no one tell you that.