Head First 2D Geometry

CHAPTER 4

your geometry toolbox

Your Geometry Toolbox

You’ve got Chapter 4 under

your belt and now you’ve

added properties of triangles

to your toolbox. For a complete

list of tool tips in the book, head to

www.headfirstlabs.com/geometry.

202 Chapter 4

Area =

1/2 base x height

T

he height comes f

rom the

a

lti

tude of the tr

iangle—i

t’s

p

er

pendicul

ar t

o the base.

Per

imeter

Add up a

ll the sides of

y

our shap

e to find the

p

er

imeter.

Hero’s f

ormul

a

Area = s (s-a) (s - b) (s - c)

s =

a + b + c

2

F

ind the semip

erimeter fir

s

t,

then use i

t t

o find ar

ea.

Centroid

T

he p

oint where a

ll thr

ee

medians of the triangle

intersec

t

Median

A line f

rom the midp

oint of a

side t

o the opposite vertex

Incenter

T

he c

enter of a circle drawn

inside the tr

iangle, t

ouching

ev

ery side

Circumcenter

T

he c

enter of a circle drawn

outside the tr

iangle, through

ev

ery v

ertex

Orthoc

enter

The point wher

e all thr

ee

altitudes of the tr

iangle

inter

sec

t

their own TV s

how in

Head F

ir

s

t Algebra.

triangle properties

TriangleGrid Solution

Stop the press! Your buddies have secretly lined up

a headline act for the festival. Solve the grid puzzle

to be the rst to know who the mystery band are.

Mys

tery band

1

T A

P Y T H A G O R E A N

2

C O M P L E M

E N R Y

3

C A

J U L Y

4

V E R

T I L A N G L E S

5

S I M I L A R

6

F A C T O R T R E E

7

A L T

I T U D E

8

C O N

G R U E N T

9

S U P L E MP

E N T A R Y

10

ET R I P L S

11

H Y P O T E N U S E

Clues

1. You use this theorem to find missing sides of right triangles.

2. Two angles adding up to 90º are known as this.

3. The month before August.

4. When lines cross, they create equal and opposite pairs of these.

5. If shapes have equal angles but are different sizes they are this.

6. To find common factors you can use one of these.

7. The height of a triangle is the length of this.

8. If triangles are similar and the same size then they are this.

9. Two angles adding up to 180º are know as this.

10. 3-4-5 and 5-12-13 are examples of Pythagorean ........

11. The longest side of a right triangle is called this.

Are y

ou a P

ajama Dea

th

f

an? Help the band get

you are here 4 203