**Theorem 1.14**: If a
line is
perpendicular to
one of two parallel
lines then it is
perpendicular to
the other.

**Proof**: If a line
is perpendicular
to one of two parallel
lines then its
slope is the negative
reciprocal of the slope of
the one parallel line.
Since parallel
lines have the same
slope, its
slope will then also be
the negative reciprocal of the
slope of the other
parallel
line, and so it will be
perpendicular to
the other parallel
line.