This document contains notes from a geometry lesson on examples of dilations. It discusses using different numbers of points to dilate circles and triangles and whether the images look accurate. It also covers how to reverse a dilation by using the reciprocal of the original scale factor. Examples are provided of dilating shapes and calculating scale factors. The document emphasizes that for curved figures like circles, using more points for the dilation produces a better image.

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Lesson 10: Examples of Dilations
Supplemental Practice: Page 99 #2­4
Do now:
Quiz

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How many points would you need to separate the
circle into to draw an accurate dilated image? Page 95
Use Compass

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How many points would you need to separate the
circle into to draw an accurate dilated image? Page 95
Use Compass
0°
160

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How many points would you need to separate the
circle into to draw an accurate dilated image? Page 95
0°
135

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Would you be able to produce a circle as an image
from the number of points used in this example?
Page 95

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What's wrong with using these points?

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What about this image. Does it look like a circle?

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The number of points to dilate that is enough is as
many as are needed to produce a dilated image that
looks like the original. For curved figures, like this
circle, the more points you dilate the better. The
location of the points you choose to dilate is also
important.

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How many points would you need to separate the
circle into to draw an accurate dilated image? Page 95
Using Calculations

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96

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What can we do to map this new triangle, triangle A'B'C',
back to the original triangle? Be as specific as possible!
Page 97

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Reverse a dilation by using a scale factor that
is the multiplicative inverse (reciprocal) of the
first scale

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Original Dilation scale factor: 2
Scale factor to reverse the dilation:
Original Dilation scale factor:
1
7
Scale factor to reverse the dilation:
Original Dilation scale factor:
2
3
Scale factor to reverse the dilation:

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Step 1- Measure the distances between point O and A
and point O and A'.
|OA| = 6 units
|OA'| = 2 units
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58°

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Step 2- Compare the distances and create an equation!
You want to now go from A' to A.
|OA| = 6 units
|OA'| = 2 units
Think!!! What can I multiply |OA'| by to give me
|OA|.

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If a figure is dilated by scale factor, r, to bring the
dilated figure back to the original size, we must
dilate it by a scale factor of .
If a scale factor is r =6, then the bring a dilated
figure back to the original size, the scale factor
must equal ________.
(reciprocal)

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Page 97

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Page 98

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Page 98

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99

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