Chapter Three

Building Geometry Solid

Incidence Axioms

I-1: For every point P and for every point Q not equal to P there exists a unique line l incident with P and Q.I-2: For every line l there exist at least two distinct points that are incident with l.I-3: There exist three distinct points with the property that no line is incident with all three of them.

Betweenness Axioms (1)

B-1 If A*B*C, then A,B,and C are three distinct points all lying on the same line, and C*B*A.

B-2: Given any two distinct points B and D, there exist points A, C, and E lying on BD such that A * B * D, B * C * D, and B * D * E.

B-3: If A, B, and C are three distinct points lying on the same line, then one and only one of the points is between the other two.

P-3.1: For any two points A and B:

Def: Let l be any line, A and B any points that do not lie on l. If A = B or if segment AB contains no point lying on l, we say A and Be are on the same sides of l.

Def: If A B and segment AB does intersect l, we say that A and B are opposite sides of l.

}AB{BAAB . and ABBAAB. iii

Betweenness Axioms (2)B-4: For every line l and for any three points A,

B, and C not lying on l: (i) If A and B are on the same side of

l and B and C are on the same side of l, then A and C are on the same side of l.

(ii) If A and B are on opposite sides of l and B and C are on opposite sides of l, then A and C are on the same side of l.

Corollary (iii) If A and B are on opposite sides of l and B and C are on the same side of l, then A and C are on opposite sides of l.

P-3.2: Every line bounds exactly two half-planes and these half-planes have no point in common.

P-3.3: Given A*B*C and A*C*D. Then B*C*D and A*B*D.

Corollary: Given A*B*C and B*C*D. Then A*B*D and A*C*D.

P-3.4: Line Separation Property: If C*A*B and l is the line through A, B, and C (B-1), then for every point P lying on l, P lies either on ray or on the opposite ray . AB AC

AB

Pasch’s Theorem

If A, B, C are distinct noncollinear points and l is any line intersecting AB in a point between A and B, then l also intersects either AC or BC. If C does not lie on l, then l does not interesect both AC and BC.

Def: Interior of an angle. Given an angle CAB, define a point D to be in the interior of CAB if D is on the same side of as B and if D is also on the same side of as C.P-3.5: Given A*B*C. Then AC = ABBC and B is the only point common to segments AB and BC.P-3.6: Given A*B*C. Then B is the only point common to rays and , and P-3.7: Given an angle CAB and point D lying on line . Then D is in the interior of CAB iff B*D*C.

ACAB

BA CB .ACAB

BC

P3.8: If D is in the interior of CAB; then: a) so is every other point on ray except A; b) no point on the opposite ray to is in the interior of CAB; and c) if C*A*E, then B is in the interior of DAE. Crossbar Thm: If is between and

, then intersects segment BC.

AD

AD ACADAB

AD

A ray is between rays and if and are not opposite rays and D is interior to CAB. The interior of a triangle is the intersection of

the interiors of its three angles. P-3.9: (a) If a ray r emanating from an

ex-terior point of ABC intersects side AB in a point between A and B, then r also intersects side AC or side BC.(b) If a ray emanates from an interior point of ABC, then it intersects one of the sides, and if it does not pass through a vertex, it intersects only one side.

AD BACA

BACA

Congruence Axioms

.

C-1: If A and B are distinct points and if A' is any point, then for each ray r emana-ting from A' there is a unique point B' on r such that B' = A' and AB A'B'.

C-2: If AB CD and AB EF, then CD EF. Moreover, every segment is congruent to itself.

C-3: If A*B*C, A'*B'*C', AB A'B', and BC B'C', then AC A'C'.

C-4: Given any angle BAC (where by defini-tion of "angle” AB is not opposite to AC ), and given any ray emanating from a point A’, then there is a unique ray on a given side of line A'B' such that B'A'C' = BAC.

B'A'C'A'

C-5: If A B and A C, then B C. Moreover, every angle is con-gruent to itself.

C-6: (SAS). If two sides and the included angle of one triangle are congruent respec-tively to two sides and the included angle of another triangle, then the two triangles are congruent.

Cor. to SAS: Given ABC and segment DE AB, there is a unique point F on a

given side of line such that ABC DEF.

DE

Propositions 3.10 - 12 P3.10: If in ABC we have AB AC,

then B C. P3.11: (Segment Substitution): If

A*B*C, D*E*F, AB DE, and AC DF, then BC EF.

P3.12: Given AC DF, then for any point B between A and C, there is a unique point E between D and F such that AB DE.

Definition:

AB < CD (or CD > AB) means that there exists a point E between C and D such that AB CE.  

Propositions 3.13

P3.13: (Segment Ordering): (a) (Trichotomy): Exactly one

of the following conditions holds: AB < CD, AB CD, or AC > CD;

(b) If AB < CD and CD EF, then AB < EF;

(c) If AB > CD and CD EF, then AB > EF;

(d) (Transitivity): If AB < CD and CD < EF, then AB < EF.

Propositions 3.14 - 16

P3.14: Supplements of congruent angles are congruent.

P3.15: (a) Vertical angles are congruent to each other.

(b) An angle congruent to a right angle is a right angle. 

P3.16: For every line l and every point P there exists a line through P perpen dicular to l.

Propositions 3.17 - 19 P3.17: (ASA Criterion for Congruence):

Given ABC and DEF with A D, C F, and AC DF. Then ABC DEF.

  P3.18: If in ABC we have B C, then

AB AC and ABC is isosceles.   P3.19: (Angle Addition): Given

between and , between and , CBG FEH, and GBA HED. Then ABC DEF.

Proposition 3.20

P3.20: (Angle Subtraction): Given between and , between and , CBG FEH, and ABC DEF. Then GBA HED.

Definition: ABC < DEF means there is a ray between

and such that ABC GEF.

Proposition 3.21 Ordering Angles P3.21: (Ordering of Angles): (a) (trichotomy): Exactly one of the

following conditions holds: P < Q, P Q, P > Q (Q < P); (b) If P < Q, and Q R, then P < R; (c) If P > Q, and Q R, then P > R; (d) If P < Q, and Q < R, then P < R.

Propositions 3.22 - 23

P3.22: (SSS Criterion for Congruence): Given ABC and DEF. If AB DE, and BC EF, and AC DF, then ABC DEF.

  P3.23: (Euclid's 4th Postulate): All

right angles are congruent to each other.