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Head First 2D Geometry by Dawn Griffiths, Stray

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If this is a
…this is a
lso a
r
ight angle.
finding missing angles
Right angles often come in pairs
A right angle is a quarter turn, so two right angles add up to one
half turn—a 180º on your skateboard, assuming you land fakey
(facing the opposite way to when you started).
Slide rail
90º turn
90º turn
Skateboard
Although it doesn’t look like one, a
straight line is still an angle—exactly a
Two right
angles sit
T
he angle on a s
traight
line is a half-turn.
180º
half turn, or two quarter turns.
together on a
straight line.
r
ight angle…
you are here 4 11
Find the next mystery angle here…
BULLET
PA
TH
MICKY
BENNY
FOOTPRINTS
18º
CRIME SCENE SKETCH
CASE GEO180
(ANGLES AS MEASURED
BUT DRAWING NOT TO
SCALE)
81º
86º
72º
PLAN VIEW
BULLET
PA
TH
sharpen solution
Find the next mystery angle here…
So the total angle made by these two
angles must also be a right angle.
? + 81º = 90º
? = 9º
MICKY
BENNY
FOOTPRINTS
18º
CRIME SCENE SKETCH
CASE GEO180
(ANGLES AS MEASURED
BUT DRAWING NOT TO
SCALE)
81º
86º
72º
PLAN VIEW
81º
Often drawing a mini-sketch of
just the part you’re working out
makes things much clearer.
This is a right angle.
12 Chapter 1
finding missing angles
Great. Whatever. But what on earth
has working out the angle behind the
corner of the building got to do with
proving that Benny shot Micky all the
way over on the other side?
Often you need to solve the puzzle piece
by piece.
Each individual angle on the sketch doesn’t tell you
much, but together they make up an accurate picture
that shows you how every part fits in relation to the other
parts.
It would be great to jump straight to the most important
angle—if you even knew what that was—but usually we
need to find a bunch of less exciting stuff to help us get
there.
It’s like any other puzzle solving really—you’re not
always fighting the end-of-level boss, sometimes you’re
just collecting the power-ups you need to beat him/her/
it when you get there!
you are here 4 13
Y
ou can assume tha
t
a
ll the lines shown
81º
We know that you can always fit a pair of right angles on a
straight line, but what if we don’t have right angles?
What might this pair of mystery angles (a and b) from our
crime scene sketch add up to?
her
e ar
e s
traight.

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