1. Answers vary with Cramer's Rule...Why?

Hello -

I have solved the following for x1 and x2 with z = 0.005 for b1=1 and b2=3. then again for b1=1.1 and b2=3. The answers vary by quite a bit for such a small change (0.5 to -2.495). can anyone tell me why this would be? thanks!

$
\begin{bmatrix}
1+3z & 1-3z \\
3-z & 3+z \end{bmatrix}
\begin{bmatrix}
x1 \\
x2 \end{bmatrix}
=
\begin{bmatrix}
b1 \\
b2 \end{bmatrix}
$

2. When you solve a pair of equations like these, you are, from the geometrical point of view, finding the co-ordinates of the point of intersection of two straight lines.
If the lines are very nearly parallel, as these are, a very small change in the slope of either of the lines can cause a large movement in the point of intersection and hence a large movement in the solution of the two equations.