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    Copyright
    Dedication
    Advance Praise for Head First 2D Geometry
    Praise for other Head First books
    The Authors
    How to Use this Book: Intro
    Ch. 1. Finding Missing Angles: Reading Between the Lines
    There's been a homicide
    In the ballistics lab you've got to cover all the angles
    Do the angles between Benny, Micky, and the bullet match up?
    Right angles aren't always marked with numbers
    Angles can be made up of other, smaller angles
    Complementary angles always add up to a right angle (90°)
    Right angles often come in pairs
    Angles on a straight line add up to 180°
    Pairs of angles that add up to 180° are called supplementary angles
    Vertical angles are always equal
    The corner angles of a triangle always add up to a straight line
    Find one more angle to crack the case
    Something doesn't add up!
    If it doesn't all add up, then something isn't as it seems
    You've proved that Benny couldn't have shot Micky!
    We've got a new sketch—now for a new ballistics report
    We need a new theory
    Work out what you need to know
    Tick marks indicate equal angles
    Use what you know to find what you don't know
    The angles of a four-sided shape add up to 360°
    Parallel lines are lines at exactly the same angle
    Parallel lines often come with helpful angle shortcuts
    Great work—you cracked the case!
    Your Geometry Toolbox
    Ch. 2. Similarity and Congruence: Shrink to Fit
    Ch. 3. The Pythagorean Theorem: All the Right Angles
    Ch. 4. Triangle Properties: Between a Rock Show and a Triangular Place
    Ch. 5. Circles: Going Round and Round
    Ch. 6. Quadrilaterals: It's Hip to be Square
    Ch. 7. Regular Polygons: It's All Shaping Up
    Index
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    Chapter 1. Finding Missing Angles: Readi... > Something doesn't add up! - Pg. 23

    finding missing angles Something doesn't add up! Using the two different methods for finding the mystery angles gives us two different results. And vertically opposite angles are equal, so ? = 18º Angles in a triangle add up to 180º so 81º + 86º + ? = 180º ? = 180º - 81º - 86º = 13º This is not good. The whole investigation could be compromised by bad math!