Measuring Angles 1-3B

1
Measuring Angles 1-3B

• What is a protractor and why is it used?

2
Vocabulary

• Ray part of a line. It has one endpoint and
  extends infinitely in one direction.

D O G
To name a ray, start with the endpoint followed
by another point on the ray.
P O N
M
3
Vocabulary

• Angle is formed by two rays (that are not
  collinear) with a common endpoint or vertex. The
  two rays are the sides of the angle.

An angle is named by using three different
letters. The vertex letter is always in the
middle. When there are no other angles around,
the angle may be named by the letter at the
vertex.
R
side
S
vertex
side
T
4
Name the angle in four ways.
A
1
B
C
5
Measuring Angles

• Angles have different sizes and are usually
  measured in degrees.
• Angles measure between 0 and 180.
• The measure of an angle is written
• The size of an angle is not the same thing as the
  angle itself.

6
Protractor
7
Using a Protractor to measure
8
Using a protractor to measure
9
More Vocabulary

• A right angle is an angle that measures 90.
• Right angles are marked with a small box.
• Two lines that intersect at right angles are
  called perpendicular lines. The symbol for
  perpendicular is

10
Short Cuts

• Sketch a line and a point not on the line (an
  inch or so away from the line). Draw several
  segments from the point to the line including the
  segment you think is the shortest one possible.
• Using a ruler to measure the segments, identify
  the shortest segment from the point to the line.
• Use a protractor to measure the angle formed by
  the line and the shortest segment.
• What do you see with the shortest segment from a
  point to a line?

11
First Theorem

• The shortest segment from a point not on a line
  to the line is the perpendicular segment.

Shortest segment
12
Assignment 1-3B

• p. 49, 1-17, 27-33