Download Lesson 18: Lines and Angles

Bell Work:
Estimate the sum of 3879 and
5276.
Answer:
9000
Lesson 18:
Lines and Angles
Line*: a straight path that extends
without end in both directions.
Ray*: a part of a line that begins at
a point and extends without end in
one direction.
Segment*: a part of a line with two
endpoints.
Can we change the order of the letters
we use to name lines and segments?
Can we change the order of the letters
we use to name rays?
Answer:
In a line and a segment, the letter
order can be changed.
In a ray it has only one endpoint
and the name of a ray begins with
the endpoint so the order matters.
Plane*: a flat surface that extends
without end.
Two lines on the same plane either
intersect or do not intersect. If the lines
do not intersect, then they are parallel
and remain the same distance apart.
Lines that intersect and from square
corners are perpendicular.
Lines that are in different planes
that do not intersect are skew lines.
For example, a wall and a ceiling
represent different planes. A line
on a wall and a line on the ceiling
that do not intersect are skew lines.
Intersecting lines form angles. An
angle is two rays with the same
endpoint. The endpoint is the
vertex and the rays are sides of the
angle.
Skew Lines
Vertex and angle
We can name angles in a variety of
ways. If we use three letters, the
middle letter is the vertex. We can
name an angle by a single letter at
the vertex. Sometimes we use a
number to name an angle.
Angle YRD or <YRD
Angle 54 or <54
Angle O or <O
We can measure angles in degrees.
A full turn is 360 degrees or 360°,
so a quarter turn is 90°.
Angles are commonly described as
acute, right, obtuse, or straight
depending on their measure. Right
angles are sometimes indicated by a
small square.
Can you name each angle?
If two angles together form a
straight angle, the angles are called
a linear pair and the sum of their
measures is 180°. <ACD and
<DCB are a linear pair.
A protractor is a tool used to measure
angles. To measure an angle we center
the protractor on the vertex of the
angle and position one of the zero
marks on one side of the angle. We
find the measure of the angle where
the other side of the angle passes
through the scale.
HW: Lesson 18 #1-30
Due Tomorrow