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Head First 2D Geometry
book

Head First 2D Geometry

by Stray (Lindsey Fallow), Dawn Griffiths
November 2009
Beginner content levelBeginner
368 pages
8h 55m
English
O'Reilly Media, Inc.
Content preview from Head First 2D Geometry
6 Chapter 1
right angles
Right angles aren’t always marked with numbers
A right angle is an internal angle (the smallest angle) between
two lines equal to 90º. Instead of drawing a curve, like we do
for most angles, we mark it with a square corner.
A right angle is a quarter turn—like the angle between the
hands on the clock at 3 o’clock or the amount you have to
turn a skateboard to get into and out of a boardslide without
landing on your face.
Lines that meet at a right angle are perpendicular
Whether they actually cross each other or only meet
at a point, two lines that form a right angle are called
perpendicular lines. We could also say that one of
the lines is perpendicular to the other.
So it turns out that we do have five
angles on the sketch after all.
Although they don’t say “90º” on them, the little square
angle marks tell us that some of the angles are right angles.
But what about all those other angles that aren’t marked
on the sketch?
This means
the same as this.
Perpendicular line
segments that meet
Perpendicular
lines segments
that cross
It is still possible to land on your face
even if you make the 90º turn.
Slide rail
90º turn
90º turn
Skateboard
90º
Line segments are parts of lines
In geometry jargon, a line goes on
forever in both directions. A line segment is
just a bit of a line with a start and an end.
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Publisher Resources

ISBN: 9780596808365Errata Page