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4.9
Definition (Reflections and Rotations.)
We now define a family of functions from
to
. If
, we define
Each of the eight functions just defined carries every box to another box with
the
same area. You should be able to see from the picture that
We can see this analytically as follows:
I will usually omit the analytic justification in cases like this.
Each of the eight functions described in definition 4.9
carries
the
square
to itself.
4.12
Definition (Symmetry of the square.)
The eight functions defined in equations (
4.10)-(
4.11)
are called
symmetries of the square.
4.13
Exercise.
Let
be the set shown in the figure. On one set of axes draw the sets
and
(label the four sets). On
another set of axes draw and label the sets
and
.
4.14
Example.
Let
and let
From the picture it is clear that
. An analytic proof of this result
is
as follows:
To show that
, I need to show that (
4.16)
implies
(
4.15). This follows because
In exercise
2.18
you assumed that
and
have the same area. In
general we will assume that if
is a set and
is a symmetry of
the square,
then
and
have the same area.
(Cf. Assumption
5.11.)
4.17
Definition (Translate of a set.)
Let
be a set in
and let
. We define the set
by
Sets of the form
will be called
translates of .
4.18
Example.
The pictures below show some examples of translates. Intuitively each
translate of
has the same shape as
and each translate of
has the
same
area as
.
4.19
Example (Translates of line segments.)
Let
.
If
, then
In particular
, so any segment can be
translated
to a segment with
as an endpoint.
4.20
Exercise.
Let
be real numbers with
and
. Show
that
if the four question marks are replaced by suitable expressions. Include some
explanation for your answer.
4.21
Exercise.
Let
be the set shown in the figure below.
a) Sketch the sets and .
b) Sketch the sets
and
,
where
is defined as in definition 4.9
Next: 4.3 The Pythagorean Theorem
Up: 4. Analytic Geometry
Previous: 4.1 Addition of Points
  Index
Ray Mayer
2007-09-07