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

?º

105º

93º

101º

?º

32º

?º

56º

62º

?º

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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