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Head First 2D Geometry by Dawn Griffiths, Stray

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this one…
this one, t
oo!
Vertical angles are always equal
When two straight lines cross they always create two pairs of
equal opposite angles, called vertical angles.
Each angle has to form a supplementary pair with either of
the angles on the other side of it, so they must be equal.
finding missing angles
Save yourself a ton of
math by spotting equal
pairs of vertical angles.
These two
are equal.
These two are
also equal.
So if y
ou know
…y
ou a
lready know
Two straight
lines cross
Add
ei
ther t
o
the
or
igina
l
angle and y
ou get 180º.
Pick one
angle
The supplements
must be equal
Find its
supplements
&
&
T
his is one
supplement…
…and
here’s the
other one.
Patterns in geometry aren’t just coincidence
These relationships are how geometry works…and to see a
really freaky one, try this:
Do this!
you are here 4 19
On some scrap paper, draw a triangle using a ruler.
Cut the triangle out.
Tear the corners off.
And then put the three corner pieces together with
the points in the middle.
Weird, huh?

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