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Lesson 2
Angles, Polygons, Triangles, Quadrilaterals
Lines
intersecting lines
point of intersection
parallel lines
Perpendicular Lines
4 right angles
straight angle
Angles
• acute angle: angle smaller than a right
angle
• obtuse angle: angle larger than a right
angle, but smaller than a straight angle
• right angle: 90 degrees
• straight angle: 180 degrees
• circle: 360 degrees
Polygons
• simple, closed, flat figures whose sides
are line segments
• named according to the number of sides
they have
• each segment of a polygon is called a side
• each endpoint of a side is called a vertex
• # of sides = # of vertices
Naming Polygons
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3 sides:
4 sides:
5 sides:
6 sides:
7 sides:
8 sides:
9 sides:
10 sides:
11 sides:
12 sides:
Concave vs. Convex
• Best shown with a picture:
concave
convex
Regular Polygons
• equilateral polygon: all sides the same length
• equiangular polygon: all angles the same
• regular polygon: all sides and angles the same
Triangles
• The sum of the measures of the angles in
a triangle is 180 degrees.
• right triangle: triangle with one right angle
• acute triangle: triangle with all angles less
than 90 degrees
• obtuse triangle: triangle with one angle
greater than 90, but less than 180
• isosceles triangle: triangle with two equal
sides
Triangles (cont.)
• the lengths of the sides of a triangle and
the measures of the angles opposite these
sides are related
• the angles opposite sides of equal lengths
have equal measures
• sides opposite angles of equal measure
have equal lengths
Example 2.1
• Find x.
130
x
30
Example 2.2
• Find x and y
y
x
50
Example 2.3
• Find x and y
x
8
y
110
8
Quadrilaterals
• parallelogram: quadrilateral with two pairs
of parallel sides
• trapezoid: quadrilateral with one pair of
parallel sides
• rectangle: quadrilateral with four right
angles
• rhombus: equilateral parallelogram
• square: rhombus with four right angles