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Head First 2D Geometry by Dawn Griffiths, Stray

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the same as this.
P
erp
endicular line
segments tha
t meet
P
erp
endicul
ar
lines segments
that cross
It is s
till p
ossible t
o l
and on y
our f
ac
e
ev
en if y
ou make the 90º turn.
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.
90º
T
his means
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.
Slide rail
90º turn
90º turn
Skateboard
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?
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.
Chapter 1 6

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