Theorem 3.1.2. Alternate Interior Angles.
If the alternate interior angles formed by a transversal of two lines are equal, then the lines are parallel.
Section 3.1 Equivalent Parallel Postulates
Each of the following is an equivalent Euclidean postulate.
- (Playfair) Given a line and a point not on that line, there exists exactly one line through that point parallel to the given line.
- (Equidistance) Lines that are parallel are everywhere equidistant.
- (Euclid) Given two lines and a transversal of those lines, if the sum of the angles on one side of the transversal is less than two right angles then the lines meet on that side.
Subsection 3.1.1 Preparation
These theorems do not require a parallel postulate.
Theorem 3.1.3. Point to line distance.
The (shortest) distance between a point and a line is from the point to the foot of the perpendicular.
Theorem 3.1.4. Convenient Euclid Parallel Axiom.
Given two lines and a transversal of those lines, if the sum of the angles on one side of the transversal is equal to two right angles then the lines are parallel.
Subsection 3.1.2 Equivalency
The following theorem produces an easier to use version of Euclid’s postulate.
Theorem 3.1.5.
Euclid’s postulate and Theorem Theorem 3.1.4 are equivalent to the following. "The sum of the angles on one side of a transversal of two lines is equal to the sum of two right angles if and only if the lines are parallel."
Theorem 3.1.6.
Euclid’s postulate implies Playfair’s axiom.
Theorem 3.1.7.
Playfair’s axiom implies the alternate interior angle converse theorem.
The alternate interior angle converse theorem states ``Given parallel lines and a transversal of those lines, the alternate interior angles formed by the transversal are congruent.’’
Theorem 3.1.8.
Playfair and the alternating interior angle converse theorem imply the equidistance of parallel lines.
Theorem 3.1.9.
The equidistance of parallel lines implies Euclid’s postulate.
Theorem 3.1.10.
Prove the three postulates are all equivalent.
