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Topic: Geometry Question #3
Replies: 39   Last Post: Jun 18, 2009 5:05 PM

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 ElizWaite@aol.com Posts: 543 Registered: 12/3/04
Re: Geometry Question #3
Posted: Jun 17, 2009 8:09 PM
 att1.html (8.0 K)

I would hope that everyone taught a dilation of -1.? It's a fairly common thing to show in high school (and I would think in middle school...aren't transformations in the 8th grade pi's and possibly 6th grade?).? I would say if someone out there is NOT teaching this, please show it!? It certainly will lead to an interesting discussion in your classroom, won't it?
Liz Waite

-----Original Message-----
From: Virginia Kuryla <VKuryla2@williamsoncentral.org>
To: nyshsmath@mathforum.org
Sent: Wed, Jun 17, 2009 1:58 pm
Subject: Re: Geometry Question #3

Ok, help me out here. And please forgive my ignorance...

I'm trying to follow your discussion. I have not seen the test yet. I work in
a middle school but am intrigued by your discussion. Question #3 would be a
multiple choice question right? From the thread I get the idea it was something
like "identify the transformation shown below" and the picture was one that was
a rotation of 180 degrees.

Though I personally have never heard of a Dilation of -1, for the moment I'm
willing to accept that this is possible. I'm curious how many of you actually
taught that to your students. If you didn't teach it that way then I have a
feeling that we are getting more worked up than the kids. It seems unlikely that
many students would have come across Dilation of -1 on their own. Unfortunately
we have no way of knowing what a student was thinking when they chose their
answer since it was multiple choice. On the other hand if Dilation of -1 is
accepted math notation and it was taught that way, I think that those districts
who had lots of students interpret it that way should contact state ed.

I'm wondering if the person who initiated this thread would share if this was a
concern that a student brought to them or a concern that they had on their own.
And what percentage of the students who answered incorrectly chose dilation.

Ginny Kuryla

>>> "George Reuter" <Reuterg@canandaiguaschools.org> 6/17/2009 10:34 AM >>>
Dolores,

With respect, regentsprep.org isn't the only source of mathematically valid
information. I could quote various sources (mathworld.com, icoachmath.com,
etc.) that would say that a dilation could have a factor of 1 or -1 (even though
we recognize these would be trivial dilations).

I do, however, agree that there's a lot of guesswork a student had to do to
answer the question, and that the guesswork that would have led to "rotation"
(realizing that ABC is congruent to A'B'C') could just as easily led that
student to "dilation" (although the savvy student probably picked rot
ation
because it was choice 1).

George Reuter

>>> "Storey, Dolores" <DStorey@newlebanoncsd.org> 6/17/2009 9:32 AM >>>
Here is the information from regentsprep.org

Dilations
Topic Index | Geometry Index | Regents Exam Prep Center

A dilation is a transformation (notation ) that produces an image that is the
same shape as the original, but is a different size. A dilation stretches or
shrinks the original figure.

The description of a dilation includes the scale factor (or ratio) and the
center of the dilation. The center of dilation is a fixed point in the plane
about which all points are expanded or contracted. It is the only invariant
point under a dilation.

A dilation of scalar factor k whose center of dilation is the origin
may be written: Dk (x, y) = (kx, ky).
If the scale factor, k, is greater than 1, the image is an enlargement (a
stretch).
If the scale factor is between 0 and 1, the image is a reduction (a shrink).
(If the scale factor should be less than 0, a dilation has occurred as well as a
reflection in the center.)

Properties preserved (invariant) under a dilation:
1. angle measures (remain the same)
2. parallelism (parallel lines remain parallel)
3. colinearity (points stay on the same lines)
4. midpoint (midpoints remain the same in each figure)
5. orientation (lettering order remains the same)
- ---------------------------------------------------------------
6. distance is NOT preserved (NOT an isometry)
(lengths of segments are NOT the same)
Dilations create similar figures.

Definition: A dilation is a transformation of the plane, , such that if O is a
fixed point, k is a non-zero real number, and P' is the image of point P, then
O, P and P' are collinear and .
Notation:

Examples:1.

P' is the image of P under a
dilation about O of ratio 2.
OP' = 2OP and

2.
is the image of under a dilation about O of ratio .

Most dilations in coordinate geometry use the origin, (0,0), as
the center of
the dilation.

Example 1:

PROBLEM: Draw the dilation image of triangle ABC with the center of dilation
at the origin and a scale factor of 2.
OBSERVE: Notice how EVERY coordinate of the original triangle has been
multiplied by the scale factor (x2).

HINT: Dilations involve multiplication!

Example 2:

PROBLEM: Draw the dilation image of pentagon ABCDE with the center of dilation
at the origin and a scale factor of 1/3.
OBSERVE: Notice how EVERY coordinate of the original pentagon has been
multiplied by the scale factor (1/3).

HINT: Multiplying by 1/3 is the same as dividing by 3!

For this example, the center of the dilation is NOT the origin. The center of
dilation is a vertex of the original figure.

Example 3:

PROBLEM: Draw the dilation image of rectangle EFGH with the center of dilation
at point E and a scale factor of 1/2.
OBSERVE: Point E and its image are the same. It is important to observe the
distance from the center of the dilation, E, to the other points of the figure.
Notice EF = 6 and E'F' = 3.

HINT: Be sure to measure distances for this problem.

- --------------------------------------------------------------------------------

Topic Index | Geometry Index | Regents Exam Prep Center
Created by Donna Roberts
Copyright 1998-2009 http://regentsprep.org ( http://regentsprep.org/ )
Oswego City School District Regents Exam Prep Center

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