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Similar Triangles I
Prepared by Title V Staff:Daniel Judge, Instructor
Ken Saita, Program Specialist
East Los Angeles College
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In geometry, two polygons are similar when one is a replica (scale model) of the other.
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Consider Dr. Evil and Mini Me from Mike Meyers’ hit movie Austin Powers. Mini Me is supposed to be an exact replica of Dr. Evil.
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Triangles are a class of polygons in geometry. Therefore we can talk about triangles that are similar. The following is a picture of similar triangles.
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Note: One triangle is a scale model of the other triangle.
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Q: How do we truly know that the above two triangles are similar (scaled model)?
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Answer– We must take a closer look at the sides of our triangles. The following conditions must all be satisfied.
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1.
2.
3.
AB=K
XY
BC=K
YZ
AC=K
XZ
Scaling Factor
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This can all be summarized as:
AB =
XY
BC =
YZ
AC = K,
XZScaling factor
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Our problem becomes as follows:
12 8 10 = = = K,
6 4 5Scaling factor
This tells us that ABC and XYZ are similar.
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Q: Can these triangles be similar?
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Answer—Yes, right triangles can also be similar but use the criteria.
AB =
XY
BC =
YZ
AC = K
XZ
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6 8 10 = = = K
4 6 8
AB =
XY
BC =
YZ
AC = K
XZ
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6 8 10 = = = K
4 6 8
6 8 = 1.5 but = 1.3
4 6
This tells us our triangles are not similar. You can’t have two different scaling factors!
Do we have equality?
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Q: The two triangles below are known to be similar, determine the missing value X.
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Answer– Using the fact that our triangles are similar . . .
7.5 4.5 = X = 3
5 X
The missing side has a length that’s 3 units. The picture should look like this . . .
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Q: The following triangles are similar. Can you determine the missing sides X and Y?
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Answer– Using the criteria,
10 Y 6 10 = = , but = 2 !
5 4 X 5
YSo, = 2
4 Y = 8
6and = 2
X X = 3
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Our triangles should look like this:
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Let’s take a closer look at the criteria that tells us when triangles are similar:
AB =
XY
BC =
YZ
AC = K
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Mathematicians find this next relationship useful as well.
AB XY AB XY AC XZ = , = , =
BC YZ AC XZ BC YZ
Why?
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End of Similar Triangles I
Title V East Los Angeles College
1301 Avenida Cesar ChavezMonterey Park, CA 91754
Phone: (323) 265-8784
Email Us At:[email protected]
Our Website:http://www.matematicamente.org
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