Activity
Assess
CONCEPT Defined Terms
In geometry, new terms are defined using previously defined or known
terms.
Description
Diagram
A segment is the part of
a line that consists of two
points, called endpoints
and all points between
them.
Notation
A
A ray is the part of a
line that consists of one
endpoint and all the
points of the line on one
side of the endpoint.
B
M
Opposite rays are rays
with the same endpoint
that lie on the same line.
⟶
​​ MN​​
N
S
An angle is formed by
two rays with the same
endpoint. Each ray is a
side of the angle and the
common endpoint is the
vertex of the angle.
T
¯
​​
AB​​
U
→
⟶
​​  and TU​​
TS​​
​​
∠Q
R
2
P
Q
∠PQR
∠2
Find Segment Lengths
EXAMPLE 1
How can you find the length of ¯
CD​​
​​  ?
The length of a segment is a positive real number. You can use the number
line to find the length of ¯
​​ CD​​.
Think about how notation and
symbols are used. How might the
notation for a segment and for
the length of a segment help you
remember their meaning?
MP.6
−3 −2 −1
B
C
0
1
D
2
3
4
The notation CD represents
the length of ¯
CD​​
​​  .
{
A
COMMUNICATE PRECISELY
There are 3 units between C and D, so CD = 3.
To find the length of a segment, count the units of length between the
endpoints. The length of ¯
CD​​
​​  is 3.
Try It! 1.Refer to the figure in Example 1. How can you find the length
of ¯
AC​​
​​  ?
6 TOPIC 1 Foundations of Geometry
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