November 2009
Beginner
368 pages
8h 55m
English
Content preview from Head First 2D Geometry
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you are here 4 7
finding missing angles
Angles can be made up of other, smaller angles
If you cut up an angle into pieces, the smaller angles
add up to the original angle.
=
+
Angle Magnets
Which of these angles matches the mystery angle?
62º 72º 45º 67º
BULLET
PATH
DOOR
MICKY
BENNY
FOOTPRINTS
BULLET FOUND
STUCK IN WALL HERE
18º
CRIME SCENE SKETCH
CASE GEO180
(ANGLES AS MEASURED
BUT DRAWING NOT TO
SCALE)
81º
86º
18º
?º
In the sketch,
this angle is made up
of these two, smaller
angles.
18º
PLAN VIEW
8 Chapter 1
angle magnets solution
Angle Magnets Solution
Which of these angles matches the mystery angle?
62º 72º 45º 67º
?º
…and this marking
means it’s a
right angle = 90º.
We know that these two angles
come together to make this angle…
18º + ?º = 90º
18º + 72º = 90º
You could have done 90–18 to find the 72º,
or you could just test all the magnets you were
given to find one that fits—making 90º.
Q:
I checked that mystery angle with a protractor and it
wasn’t 72º—so how come you’re saying it is?
A: In geometry, unless you’re specifically told to measure
an angle, assume that the drawing isn’t accurate, but that the
numbers on the sketch are. We calculate missing angles rather
than measure them.
Q:
What about all the other lines on the diagram? Do we
know if they’re straight?
A: The other line segments on the sketch re ...
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