November 2009
Beginner
368 pages
8h 55m
English
Content preview from Head First 2D Geometry
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you are here 4 17
finding missing angles
Pairs of angles that add up to 180º are
called supplementary angles
Any two angles which add up to 180º are known as
supplementary angles. They are easiest to spot when
they’re on a straight line, but they can also be far apart.
BULLET
PATH
MICKY
BENNY
FOOTPRINTS
18º
CRIME SCENE SKETCH
CASE GEO180
(ANGLES AS MEASURED
BUT DRAWING NOT TO
SCALE)
81º
86º
Use supplementary angles to find mystery angles a and b.
Is there anything surprising about mystery angle a?
72º
bº
aº
81º
bº
81º
bº
These two
angles must be
supplementary.
They would still be
supplementary even if
they weren’t touching,
but we couldn’t be sure
without knowing what
angle b is.
9º
PLAN VIEW
18 Chapter 1
that’s not a coincidence…
Use supplementary angles to find mystery angles a and b.
Is there anything surprising about mystery angle a?
BULLET
PATH
MICKY
BENNY
FOOTPRINTS
18º
CRIME SCENE SKETCH
CASE GEO180
(ANGLES AS MEASURED
BUT DRAWING NOT TO
SCALE)
81º
86º
72º
bº
aº
9º
81º
bº
These two angles are on a straight line and
supplementary, so they add up to 180º.
81º+ bº = 180º
bº = 99º
bº
aº
These two angles are on a straight line and
supplementary, so they also add up to 180º.
aº + bº = 180º
aº + 99º = 180º
aº = 81º
aº is the same as the first angle,
which is supplementary with bº!
PLAN VIEW
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