Non-Euclidean Geometry

1. Non-Euclidean Geometry Br. Joel Baumeyer, FSC Christian Brothers University

2. The Axiomatic Method of Proof • The axiomatic method is a method of proving that a conclusion is correct. • Requirements: 0. The meaning of words and symbols used in the discussion must be understood clearly by all involved in the discussion. 1. Certain statements called axioms (or postulates) are accepted without justification. 2. Agreement on certain rules of reasoning (I.e. agreeing on how and when one statement follows another).

3. Euclid’s Axiom Base

4. Terms • Defined terms - words which have common meaning to all involved in the discussion and which are subject to the rules of reasoning in requirement # 2 above. • Undefined terms - words which are not defined but to which certain properties are ascribed. For Euclidean plane geometry they are: • point, line, (plane) • lie on, between, congruent

5. Some Basic Concepts • A Set is a collection of objects. • Equal vs Congruent* • Equal (=) means identical () • Congruent( ) undefined (for the time being think “fits exactly on”) *both terms are what is known as equivalence relations

6. Euclid’s First Four Axioms (1) and necessary definitions • P1: For every point P and for every point Q not equal to P there exists a unique line l that passes through P and Q. Def: The segment AB is the set whose members are the points A and B and all points that line on the line AB and are between A and B. (A and B are endpoints.)

7. Euclid’s First Four Axioms (2) and necessary definitions • P2: For every segment AB and for every segment CD there exists a unique point E such that B is between A and E and segment CD is congruent to segment BE. • Def: Given two points O and A. The set of all points P such that segment OP is congruent to segment OA is called a circle with O as center and each segment OP is called a radius of the circle.

8. Euclid’s First Four Axioms (3) and necessary definitions • P3: For every point O and every point A not equal to O there exists a circle with center O and radius OA • Def: The ray AB is the following set of points lying on the line AB: those points that belong to the segment AB and all points C on AB such that B is between A and C. The ray AB is said to emanate from the vertex A and to be a part of line AB.

9. Euclid’s First Four Axioms (4) and necessary definitions • Def: Rays AB and AC are opposite if they are distinct, if they emanate from the same point A, and if they are part of the same line AB = AC. • Def: An "angle with vertex A" is a point A together with two distinct nonopposite rays AB and AC, called the sides of the angle emanating from A.

10. Euclid’s First Four Axioms (5) and necessary definitions • Def: If two angles BAD and CAD have a common side AD and the other two sides AB and AC form opposite rays, the angles are called supplementary angles. • Def: An angle BAD is a right angle if it has a supplementary angle to which it is congruent. • P4: All right angles are congruent to each other.

11. The Parallel Postulate • Def: Two lines l and m are parallel if they do not intersect, i.e. if no point lies on both of them. • Euclid’s Parallel Axiom: For every line l and for every point P that does not lie on l there exists a unique line m through P that is parallel to l.