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Head First 2D Geometry by Dawn Griffiths, Stray

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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.
These two
angles must be
supplementary.
81º
81º
They would still be
supplementary even if
they weren’t touching,
but we couldn’t be sure
without knowing what
angle b is.
Use supplementary angles to find mystery angles a and b.
Is there anything surprising about mystery angle a?
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
you are here 4 17
that’s not a coincidence…
Use supplementary angles to find mystery angles a and b.
Is there anything surprising about mystery angle a?
BULLET
PA
TH
MICKY
BENNY
FOOTPRINTS
18º
CRIME SCENE SKETCH
CASE GEO180
(ANGLES AS MEASURED
BUT DRAWING NOT TO
SCALE)
81º
86º
72º
81º
These two angles are on a straight line and
supplementary, so they add up to 180º.
81º+ bº = 180º
bº = 99º
These two angles are on a straight line and
supplementary, so they also add up to 180º.
aº + bº = 180º
PLAN VIEW
aº is the same as the first angle,
aº + 99º = 180º
which is supplementary with bº!
aº = 81º
18 Chapter 1

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