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Head First 2D Geometry by Dawn Griffiths, Stray

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sharpen solution
TIRE
TRACKS
x and y are supplementary
x is less than y
x is equal to y
nothing—they could be any angles
If our bullet bounced as the lab technician
desribed, what do we know should be true
about mystery angles x and y?
Tick marks indicate equal angles
Tick marks on angles are used to show that the
angles are equal, even if we don’t know what size
the angles are.
Each set of equal angles has a different number of
ticks, so you can mark more than one set on the
same sketch if you need to.
Remember—your sketch isn’t
usually accurate enough to
see by eye whether two
angles are equal.
Unless you’re told otherwise, a
sketch is just a sketch. Even if the angles drawn
look pretty close to the real angles, you can’t rely
on your eye or a protractor—you’ve gotta work
the angles out.
Things that bounce equally
bounce like this.
bounce
These tick marks on
the angles show that
they are equal.
30 Chapter 1

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