If A and B are two points on a line, any pair of points C and D on the line for which
is said to divide AB harmonically. The points C and D are then said to be harmonic conjugates with respect to A and B.
Lemma 14.1.1. Given ordinary points A and B, and given a positive integer k where k ≠ 1, there are two ordinary points C and D such that
One of the points C and D is between A and B, while the other is exterior to the segment AB.
Proof. Choose a point C on the line AB such that
Since k > 0, then CB < AB, and we may assume that C lies between A and B.
Now, we have
Now we find the point D, which will be exterior to the segment AB—beyond B if k > 1 and beyond A if 0 < k < 1.
Assuming that k > 1, ...