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Chapter Three
Building Geometry Solidly
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:
i. AB  BA  AB and ii. AB  BA  {AB}
 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.
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 halfplanes 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 AB or on the opposite ray 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 AC as B and
if D is also on the same side of AB as C.
P-3.5: Given A*B*C. Then AC = ABBC 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 BA and B C , and AB  AC.
P-3.7: Given an angle CAB and point D lying
on line BC . Then D is in the interior of CAB
iff B*D*C.
P3.8: If D is in the interior of CAB; then:
a) so is every other point on ray AD
except A;
AD
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 AD is between AC
and AB , then AD intersects segment BC.
 A ray AD is between rays A C and A B if A B
and A C 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.
.
Congruence Axioms
Congruence Axioms (1)
 C-1: If A and B are distinct points and if A'
is any point, then for each ray r emanating
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'.
Congruence Axioms (2)
C-4: Given any angle BAC (where by
defini-tion of "angle” AB is not
opposite to AC ), and given any ray A' B'
emanating from a point A’, then there
is a unique ray A' C' on a given side of
line A'B' such that B'A'C' = BAC.
C-5: If A  B and A  C,
then B  C. Moreover, every angle
is con-gruent to itself.
Congruence Axioms (3)
 C-6: (SAS).
If two sides and the included
angle of one triangle are congruent respectively 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 DE such that ABC  DEF.
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 perpendicular 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 BG between BA
and BC , EH between ED and EF ,
 CBG   FEH, and  GBA   HED.
Then  ABC   DEF.
Proposition 3.20
 P3.20: (Angle Subtraction): Given
BG between
BA and BC , EH between ED and EF,
 CBG   FEH, and  ABC   DEF.
Then  GBA   HED.
 Definition:
  ABC <  DEF means there is a ray EG
between ED and EF 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, or  P >  Q
(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.