**Theorem 3.11**: The
line joining the two
points where two
circles intersect is
perpendicular to
the line joining their
centers.

**Proof**: If the
line joining the centers
is horizonal, then by
Theorem 3.10, the
line containing the
centers is vertical. If not, then by Theorem
3.10, the slope of
the line containing the
solutions is

which is the negative recoprocal of the slope of the line joining the centers.