Head First 2D Geometry

table of contents

Table of Contents (Summary)

Intro xvii

1 Finding missing angles: Reading between the lines 1

2 Similarity and congruence: Shrink to fit 49

3 The Pythagorean Theorem: All the right angles 103

4 Triangle properties: Between a rock show and a triangular place 149

5 Circles: Going round and round 205

6 Quadrilaterals: It’s hip to be square 235

7 Regular polygons: It’s all shaping up 273

Table of Contents (the real thing)

Intro

Your brain on Geometry. Here you are trying to learn something,

while here your brain is doing you a favor by making sure the learning doesn’t

stick. Your brain’s thinking, “Better leave room for more important things, like

which wild animals to avoid and whether naked snowboarding is a bad idea.”

So, how do you trick your brain into thinking that your life depends on knowing

about triangles and circles and the Pythagorean Theorem?

Who is this book for? xviii

We know what you’re thinking xix

Metacognition: thinking about thinking xxi

Here’s what YOU can do to bend your brain into submission xxiii

Read me xxiv

The technical review team xxvi

Acknowledgments xxvii

ix

?º

CHARLIE GREEN

HF COUNTY

SHERIFF’S OFFICE

table of contents

finding missing angles

Reading between the lines

1

Ever get the feeling there’s something they’re not

telling you?

If you want to master the real world, you need to get geometry. It’s a set of

tools for turning a little bit of information into a complete picture. Whether

you want to design something, build something or find out how a situation

really went down, geometry can make sure you’ve always got the lowdown.

So if you want to keep in the loop, grab your hat, pack your pencil, and join us

on the bus to Geometryville.

There’s been a homicide 2

In the ballistics lab you’ve got to cover all the angles 3

Do the angles between Benny, Micky, and the bullet match up? 4

Right angles aren’t always marked with numbers 6

Angles can be made up of other, smaller angles 7

Complementary angles always add up to a right angle (90º) 9

Right angles often come in pairs 11

Angles on a straight line add up to 180º 14

Pairs of angles that add up to 180º are called supplementary angles 17

Vertical angles are always equal 19

The corner angles of a triangle always add up to a straight line 20

18º

Find one more angle to crack the case 21

Something doesn’t add up! 23

If it doesn’t all add up, then something isn’t as it seems 24

You’ve proved that Benny couldn’t have shot Micky! 25

We’ve got a new sketch—now for a new ballistics report 26

We need a new theory 27

Work out what you need to know 29

Tick marks indicate equal angles 30

Use what you know to find what you don’t know 31

The angles of a four-sided shape add up to 360º 35

Parallel lines are lines at exactly the same angle 39

Parallel lines often come with helpful angle shortcuts 40

Great work—you cracked the case! 44

Your Geometry Toolbox 46

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