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2-5
PERIOD
Study Guide and Intervention
Postulates and Paragraph Proofs
Points, Lines, and Planes
In geometry, a postulate is a statement that is accepted
as true. Postulates describe fundamental relationships in geometry.
Postulate
Postulate
Postulate
Postulate
Postulate
Postulate
Postulate
2.1:
2.2:
2.3:
2.4:
2.5:
2.6:
2.7:
Example
Through any two points, there is exactly one line.
Through any three noncollinear points, there is exactly one plane.
A line contains at least two points.
A plane contains at least three noncollinear points.
If two points lie in a plane, then the entire line containing those points lies in the plane.
If two lines intersect, then their intersection is exactly one point.
If two planes intersect, then their intersection is a line.
Determine whether each statement is always,
sometimes, or never true.
a. There is exactly one plane that contains points A, B, and C.
Sometimes; if A, B, and C are collinear, they are contained in many planes. If they are
noncollinear, then they are contained in exactly one plane.
b. Points E and F are contained in exactly one line.
Always; the first postulate states that there is exactly one line through any two points.
Exercises
Determine whether each statement is always, sometimes, or never true.
1. A line contains exactly one point.
2. Noncollinear points R, S, and T are contained in exactly one plane.
3. Any two lines ℓ and
m intersect.
−−−
4. If points G and H are contained in plane M, then GH is perpendicular to plane
5. Planes
M.
R and S intersect in point T.
−− −−−
−−
6. If points A, B, and C are noncollinear, then segments AB, BC, and CA are contained in
exactly one plane.
−−
−− −−
−−
In the figure, AC and DE are in plane Q and AC || DE.
State the postulate that can be used to show each
F
statement is true.
Q
C
7. Exactly one plane contains points F, B, and E.
B
A
D
"#$ lies in plane
8. BE
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Lesson 2-5
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c. Two lines intersect in two distinct points M and N.
Never; the intersection of two lines is one point.
G
Q.
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Study Guide and Intervention
PERIOD
(continued)
Postulates and Paragraph Proofs
Paragraph Proofs A logical argument that uses deductive reasoning to reach a valid
conclusion is called a proof. In one type of proof, a paragraph proof, you write a
paragraph to explain why a statement is true.
A statement that can be proved true is called a theorem. You can use undefined terms,
definitions, postulates, and already-proved theorems to prove other statements true.
−−
Example
In △ABC, BD is an angle bisector. Write a
paragraph proof to show that ∠ ABD # ∠CBD.
By definition, an angle bisector divides an angle into two congruent
−−−
angles. Since BD is an angle bisector, ∠ ABC is divided into two
congruent angles. Thus, ∠ ABD $ ∠CBD.
B
A
D
C
Exercises
1. Given that ∠ A $ ∠D and ∠D $ ∠E, write a paragraph proof to show that ∠ A $ ∠E.
B
C
M
E
−−−
−−
3. Given that S is the midpoint of QP, T is the midpoint of PR,
−−
and P is the midpoint of ST, write a paragraph proof to show
that QS = TR.
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N
F
R
Q
S
P
T
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−−− −−
−−−
2. It is given that BC $ EF, M is the midpoint of BC, and N is the
−−
midpoint of EF. Write a paragraph proof to show that BM = EN.
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Skills Practice
Postulates and Paragraph Proofs
Explain how the figure illustrates that each statement
is true. Then state the postulate that can be used to show
each statement is true.
F
C
B
1. Planes O and M intersect in line r.
N
A
O
E
p
D
M
G
H
2. Line p lies in plane N.
N
J
I
q
K
L
r
s
Determine whether each statement is
always, sometimes, or never true. Explain your reasoning.
3. Three collinear points determine a plane.
4. Two points A and B determine a line.
! # and DP
! #. State
# is in plane J and H lies on DG
In the figure, DG
the postulate that can be used to show each statement is true.
6. G and P are collinear.
H
P
J
D
G
7. Points D, H, and P are coplanar.
−−
8. PROOF In the figure at the right, point B is the midpoint of AC and
−−−
point C is the midpoint of BD. Write a paragraph proof to prove that
AB = CD.
A
B
C
D
Lesson 2-5
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5. A plane contains at least three lines.
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Practice
Postulates and Paragraph Proofs
Explain how the figure illustrates that each statement
is true. Then state the postulate that can be used to show
each statement is true.
+
3
1. The planes J and K intersect at line m.
ℓ
2
2. The lines l and m intersect at point Q.
m
1
,
Determine whether the following statements are always, sometimes, or never true.
Explain.
3. The intersection of two planes contains at least two points.
4. If three planes have a point in common, then they have a whole line in common.
" lie in plane A . State the postulate
In the figure, line m and TQ
that can be used to show that each statement is true.
S
Q
L
!"# intersect at T.
6. Line m and ST
−−
−−−
7. In the figure, E is the midpoint of AB and CD, and AB = CD. Write a
−− −−−
paragraph proof to prove that AE % ED.
C
A
E
B
D
Copyright © Glencoe/McGraw-Hill, a division of The McGraw-Hill Companies, Inc.
m A
T
5. Points L, and T and line m lie in the same plane.
8. LOGIC Points A, B, and C are noncollinear. Points B, C, and D are noncollinear. Points
A, B, C, and D are noncoplanar. Describe two planes that intersect in line BC.
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