2.5 Postulates & Paragraph
Proofs
Objectives

Identify and use basic postulates about points,
lines, and planes.

Write paragraph proofs.
Postulates

In geometry, a postulate is a statement that
describes a fundamental relationship between
the basic terms of geometry.

Postulates are always accepted as true.
Postulates

Postulate 2.1 – Through any two points, there
is exactly one line.

Postulate 2.2 – Through any three points not
on the same line, there is exactly one plane.
Example 1:
SNOW CRYSTALS Some snow crystals are shaped
like regular hexagons. How many lines must be
drawn to interconnect all vertices of a hexagonal
snow crystal?
Explore The snow crystal has six vertices since a regular
hexagon has six vertices.
Plan
Draw a diagram of a hexagon to illustrate the
solution.
Example 1:
Solve
Label the vertices of the hexagon A, B, C, D,
E, and F. Connect each point with every other
point. Then, count the number of segments.
Between every two points there is exactly one
segment. Be sure to include the sides of the
hexagon. For the six points, fifteen segments
can be drawn.
Example 1:
Examine In the figure,
are all segments
that connect the vertices of the snow crystal.
Answer: 15
Your Turn:
ART Jodi is making a string art design. She has
positioned ten nails, similar to the vertices of a
decagon, onto a board. How many strings will she
need to interconnect all vertices of the design?
Answer: 45
More Postulates

Postulate 2.3 – A line contains at least two
points.

Postulate 2.4 – A plane contains at least three
points not on the same line.

Postulate 2.5 – If two points lie in a plane, then
the entire line containing those points lies in that
plane.
And Even More Postulates

Postulate 2.6 – If two lines intersect, then their
intersection is exactly one point.

Postulate 2.7 – If two planes intersect, then
their intersection is a line.
Example 2a:
Determine whether the following statement is
always, sometimes, or never true. Explain.
If plane T contains
plane T contains point G.
contains point G, then
Answer: Always; Postulate 2.5 states that if two points
lie in a plane, then the entire line containing
those points lies in the plane.
Example 2b:
Determine whether the following statement is
always, sometimes, or never true. Explain.
For
, if X lies in plane Q and Y lies in plane R,
then plane Q intersects plane R.
Answer: Sometimes; planes Q and R can be parallel,
and
can intersect both planes.
Example 2c:
Determine whether the following statement is
always, sometimes, or never true. Explain.
contains three noncollinear points.
Answer: Never; noncollinear points do not lie on the
same line by definition.
Your Turn:
Determine whether each statement is always,
sometimes, or never true. Explain.
a. Plane A and plane B intersect in one point.
Answer: Never; Postulate 2.7 states that if two planes
intersect, then their intersection is a line.
b. Point N lies in plane X and point R lies in plane Z.
You can draw only one line that contains both points
N and R.
Answer: Always; Postulate 2.1 states that through any
two points, there is exactly one line.
Your Turn:
Determine whether each statement is always,
sometimes, or never true. Explain.
c. Two planes will always intersect a line.
Answer: Sometimes; Postulate 2.7 states that if the two
planes intersect, then their intersection is a
line. It does not say what to expect if the
planes do not intersect.
Theorems

We use undefined terms, definitions, postulates,
and algebraic properties of equality to prove that
other statements or conjectures are true. Once a
statement or conjecture has been shown to be
true, it is called a theorem.

Once proven true, a theorem can be used like a
definition or postulate to justify other
statements or conjectures.
Paragraph Proofs

Proof – a logical argument in which each
statement you make is supported by a statement
that is accepted as true

Paragraph Proof – a type of proof in which
you write a paragraph to explain why a
conjecture for a given situation is true (also,
referred to as an “informal proof ”)
Paragraph Proofs
There are 5 essential parts of a good proof:
 State
the theorem or conjecture to be proven.
 List the given information.
 If possible, draw a diagram to illustrate the given
information.
 State what is to be proved.
 Develop a system of deductive reasoning.
Paragraph Proofs
Hint:
Before writing a proof, you should have a plan.
One strategy is to work backwards. Start with
what you want to prove, and work backwards
step by step until you reach the given
information.
Theorems

Theorem 2.8 (Midpoint Theorem)
If M is the midpoint of AB, then AM  MB.
Example 3:
Given
intersecting
, write a paragraph proof
to show that A, C, and D determine a plane.
Given:
intersects
Prove: ACD is a plane.
Proof:
must intersect at C because if two
lines intersect, then their intersection is exactly
one point. Point A is on
and point D is on
Therefore, points A and D are not collinear.
Therefore, ACD is a plane as it contains three
points not on the same line.
Your Turn:
Given
midpoint of
is the midpoint of
and X is the
write a paragraph proof to show that
Your Turn:
Proof: We are given that S is the midpoint of
X is the midpoint of
and
By the definition of midpoint,
Using the definition of congruent
segments,
Also using the given
statement
and the definition of congruent
segments,
If
then
Since S and X are midpoints,
By substitution,
congruence,
and by definition of
Assignment

Geometry:
Pg. 92
#12 - 27

Pre-AP Geometry:
Pg. 92
# 12 - 28