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ABCs of Geometry
“The hardest part of math last week was adding all the
decimals on the restaurant menu.” –Student 6
A is for Acute Angle
• Acute angles are less than
90 degrees.
• An equilateral triangle has
three acute angles.
• How many acute angles
does an obtuse triangle
have?
B is for Base
• The base is the bottom of a
three-dimensional figure in
geometry.
• What if the top is parallel
to the bottom?
• Can you think of an
example?
C is for Congruent
• If angles or figures are
congruent, they are the
same shape and size.
• If there are no numbers on
a picture, how can we tell if
the sides and angles are
congruent?
D is for Degree
• A degree (°) is a unit of
measure of an angle.
• The sum of all three angles
in a triangle always equals
180 degrees.
• How many degrees are in a
circle?
E is for Equilateral Triangle
• Equilateral triangles have
three equal sides and
angles.
• Each of the angles is always
60 degrees.
• If the angles are always the
same size, can the side
lengths vary?
F is for Face
• A face is a flat surface of a
three-dimensional figure.
This is one of
eight faces.
• Prisms can have many faces.
This is one
of five faces.
• How can you usually tell
how many faces a prism has
without counting?
G is for Geometry
• Geometry is the study of
two-dimensional and threedimensional figures.
• It uses relationships
between points, lines, angles
and surfaces.
• Does anyone know what
“geometry” literally means?
H is for Hypotenuse
• The hypotenuse is the side
that is opposite the right
angle in a right triangle.
• The other two sides of a
right triangle are called
opposite and adjacent.
• Can a triangle have more
than one right angle?
I is for Isosceles Triangle
• An isosceles triangle has
two equal sides.
• By default, there are also
two equal angles.
• Are isosceles triangles
always acute (like the
pictures)?
J is for Journal
• Writing is a way for
students to communicate
their understanding of
geometry.
• Writing helps connect prior
knowledge to polygons,
angles and lines.
• How do journals help both
teachers and students?
K is for Kite
• A kite is a quadrilateral
with two pairs of adjacent
sides that are congruent.
– Note the hash marks
• What must the diagonals of
a kite always be?
– Hint: look at the pink
box in the picture
L is for Line of Symmetry
• A line of symmetry divides
a geometric figure into two
congruent portions.
• A dotted line usually shows
a line of symmetry.
• Does the letter “L” have a
line of symmetry?
M is for Midpoint
• The midpoint is the point
on a line segment that
divides it into two
congruent pieces.
• A line segment can only
have one midpoint.
• Why does a line not have a
midpoint?
N is for Nonagon
• A Nonagon is a nine-sided
polygon.
• The Figure shown is a
regular nonagon (all sides
equal length).
• Are the angles all equal?
O is for Obtuse Angle
• Obtuse angles are larger
than 90°.
• Obtuse triangles can be
scalene or isosceles.
• How many obtuse angles
can an obtuse triangle have?
P is for Parallelogram
• A parallelogram is a
quadrilateral.
• It has 2 pairs of opposite
sides that are parallel.
• What can we say about the
diagonals?
Q is for Quadrilateral
• A quadrilateral is a foursided polygon.
• Polygons are closed figures
with line segments for sides.
• What is an example of a
quadrilateral?
R is for Rectangle
• A rectangle has four 90°
angles.
• The opposite sides are
congruent.
• Can a rectangle also be a
square?
S is for Supplementary Angles
• Supplementary angles are
two angles that add up to
180 degrees.
• <A + <B =180
• What is the supplementary
angle for an angle of 42°?
T is for Trapezoid
• A trapezoid is a
quadrilateral.
• It has one set of parallel
sides.
• Are all three of these
figures trapezoids?
U is for Undecagon
(also called “Hendecagon”)
• An Undecagon is a polygon
that had eleven sides.
• The figure is a regular
polygon because all the
sides are of equal length.
• Are the angles equal?
V is for Vertex
• A vertex is where two
lines/rays intersect to form
an angle.
• It is also the corners on a
polygon.
• What do the sides and
vertices have in common?
(in the figure with the red
arrow)
W is for Word Problem
• A word problem is a way to
ask math questions.
• The information in the
problem is presented in
word form rather than
mathematical notation.
• What are word problems
also called?
X is for X-axis
• The X-axis is a horizontal
line on a Cartesian graph.
• It runs through the origin
(0,0).
• How could we write an
equation for the x-axis in
y= form?
Y is for Y-axis
• The Y-axis is a vertical line
on a Cartesian graph.
• It also runs through the
origin (0,0).
• How could we write an
equation for the y-axis?
Z is for Zero Angle
.
0°
• A Zero angle is an angle
that measures 0 degrees.
• Depending on the context,
the same angle could also
be considered 360°.
• Is a zero angle (0°, not
360°) an obtuse, acute, or
right angle?
References
• www.yahoo.com
• www.math.com/school/glossary
• www.mathwords.com