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Head First 2D Geometry by Dawn Griffiths, Stray

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CHAPTER 1
your geometry toolbox
Your Geometry Toolbox
You’ve got Chapter 1 under your
belt, and now you’ve added
techniques for finding missing
angles to your toolbox. For a complete
list of tool tips in the book, visit
www.headfirstlabs.com/geometry.
A square mark indicates a
RIGHT ANGL
E (90º).
Angles can be made
up of other, smaller
angles.
Angles in a f
our-sided
shap
e add up t
o 360º.
Angles in a tr
iangle
add up t
o 180º.
Angles on a s
traight
line add up to 180º.
46 Chapter 1
finding missing angles
I
f tw
o a
n
g
l
e
s
a
dd up
t
o 1
8
0
º
,
t
he
y a
r
e
SUPPL
EME
NTA
RY.
If tw
o angles add
up t
o 90º, they ar
e
COMPLEME
NT
ARY.
P
arallel lines make rep
ea
t
sets of vertica
l angles.
F
, Z, and N pa
tterns
around para
llel lines sav
e
y
ou a ton of ma
th.
V
ertica
l angles ar
e
equal (and opp
osi
te
each other).
you are here 4 47
who does what solution
Angles in a triangle add
up to 180º
Match each technique for finding missing angles to a sketch. You can use
each technique just once, but some sketches will need more than one to
find the missing angle.
Vertical angles are equal
Parallel lines cross other
lines at the same angles
Angles on a straight line add
up to 180º
Angles in a right angle add
up to 90º
Angles in a four-sided
shape add up to 360º
73º
105º
93º
101º
32º
56º
62º
SOlUTion
?º = 180º - (56º+ 62º)
= 62º
73º
? = 73º
75º
?º = 360º -
(75º + 93º +
101º) = 91º
? = 90º - 32º
= 58º
48 Chapter 1

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