Download lesson 1.3 - James Rahn

SECTION 1-3
WHAT IS A WIDGET?
Good definitions are important in Geometry.
We’ll learn how to write good definitions in
this lesson.
Which figure is a widget in the last group?
What things do all Widgets have in common?
What things do Widgets have that others do
not have?
What makes a Widget a Widget?
A protractor is a geometric tool used to measure
angles.
1. You classify what a protractor is.
2. Then you tell how it differs from other
geometric tools.
A square is a
_______________________
that
________________________
Once you have written a definition you should
test it.
Try to look for a counterexample.
That means you should try to create a figure that
fits your definition but isn’t what you are trying to
define.
Check your definition of a square.
EXAMPLE A

Everyone knows, “A square is a figure with four
equal sides. What’s wrong with this definition?

Sketch a counterexample.

Write a better definition of a square.
WRITING A GOOD DEFINITION
Before you write a few definition here are
some special markings we use for parallel
lines.
The same number of
arrow marks indicate
parallel lines.
The || mean “is parallel
to”.
Before you write a few definition here are
some special markings we use perpendicular
lines.
The small square in
the corner of the
angle indicates that
it measures 90o.
The symbol ┴
means “is
perpendicular to”.
EXAMPLE B

Define these terms

Parallel lines

Perpendicular lines
SPECIAL NOTE

Two lines that do not intersect and are
noncoplanar are skew lines
DEFINING ANGLES
1. Write a definition for each bold faced term.
Make sure your definitions highlight important
differences.
2. Trade definitions and test each other’s
definitions for counterexamples.
3. If another group member finds a
counterexample to one of your definitions, write
a better definition. As a group, decide on the
best definition for each term.
4. As a class agree on common definitions. Add
these to your notebook. Draw and label a
picture to illustrate each definition.
RIGHT ANGLE
ACUTE ANGLE
OBTUSE ANGLE
COMPLEMENTARY ANGLES
SUPPLEMENTARY ANGLES
VERTICAL ANGLES
LINEAR PAIR OF ANGLES
EXAMPLE

Use the picture at the
right to write a definition
for a pair of linear angles
by using the names of two
angles in your definition.