A=rand(3)
cond_A=cond(A)
norm(inv(A))*norm(A)

%

cond_A_1=cond(A,1)
norm(inv(A),1)*norm(A,1)

%

b=rand(3,1)
%solve the system Ax=b
x=A\b
r=b-A*x
norm(r)

%

A(1,:)=0
det(A)
inv(A)
cond(A)
b=rand(3,1)
%solve the system Ax=b
x=A\b
r=b-A*x
norm(r)

%

A=hilb(5)
cond(A)
%solve the system Ax=b
b=rand(5,1)
x=A\b
r=b-A*x
norm(r)