November 2009
Beginner
368 pages
8h 55m
English
Content preview from Head First 2D Geometry
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you are here 4 73
similarity and congruence
Complex shapes can be similar, too
Similarity isn’t just for triangles! Provided you shrink or grow your shapes
proportionally, they can also be similar. When shapes are proportional,
the ratios between the lengths of their different lines are the same.
The ratio of length
to width in this
design is 120/56.
The ratio of length
to width in this
design is 60/28.
If you’re not a wiz at
fractions, you could check
this on a calculator.
The ratio of length
to width in this
design is 60/46.
120
56
60
28
60
46
Similar:
Not Similar:
60/28 and 120/56
are the same ratio.
60/46 and 120/56
aren’t the same ratio.
Can you use proportionality to tell if shapes
are similar even if you don’t know ANY of their
angles?
74 Chapter 2
similarity exposed
Head First: You’re really becoming popular—a lot
of people are saying you’re the time-saving technique
they wish they’d always known.
Similarity: Yes—it’s nice of you to say so! I do like
to think I’m rather, um, efficient is the best word, I
guess.
Head First: That’s certainly true! But there’s one
thing I’m wondering.…
Similarity: Go on.…
Head First: Well, people recognize you by
matching angles—and others use the proportional
thing—and I’m just wondering, which is the real you?
Similarity: I don’t understand. You mean you ...
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