Subtraction not calculating correctly

A=[2 -3; 6 3];
c=[-5; 1];
% Ax=c  -->  (inv(A)*A)*x = inv(A)*C  -->  x=inv(A)*c

X=inv(A)*c;

%Why does AX-C not equal of column vector of zeros > [0;0]?
AX=A*X
AXminusC=AX-c

hi there,

im struggling to figure out why i am getting an incorrect subtraction for my variable AXminusC.

The result of AX is a column vector [-5;1] therefore AX-c should output a matrix of [0;0]

Im struggling to understand why AXminusC outputs a matrix of 1.0e-15*[0;-0.220].

[James Tursa]

Because of floating point arithmetic rounding effects, you should not expect the calculation A*(inv(A)*c) - c to result in exactly 0's. You might get lucky in some cases and actually get exact 0's, but in the general case you should expect to get relatively small non-zero residuals from this calculation.

Answer 1

[Cris LaPierre]

You may need to clear your workspace. When I run your code, the result is [0;0]

A=[2 -3; 6 3];
c=[-5; 1];

X=inv(A)*c;
AX=A*X;

AXminusC=AX-c

AXminusC = 2×1

0 0