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SECTION 9-2
• Curves, Polygons, and Circles
Slide 9-2-1
CURVES, POLYGONS, AND CIRCLES
• Curves
• Triangles and Quadrilaterals
• Circles
Slide 9-2-2
CURVES
The basic undefined term curve is used for
describing figures in the plane.
Slide 9-2-3
SIMPLE CURVE; CLOSED CURVE
A simple curve can be drawn without
lifting the pencil from the paper, and
without passing through any point twice.
A closed curve has its starting and ending
points the same, and is also drawn without
lifting the pencil from the paper.
Slide 9-2-4
SIMPLE CURVE; CLOSED CURVE
Simple;
closed
Simple; not
closed
Not simple;
closed
Not simple;
not closed
Slide 9-2-5
CONVEX
A figure is said to be convex if, for any two
points A and B inside the figure, the line segment
AB is always completely inside the figure.
A
B
B
Convex
Not convex
A
Slide 9-2-6
POLYGONS
A polygon is a simple, closed curve made up of
only straight line segments. The line segments
are called sides, and the points at which the sides
meet are called vertices.
Polygons with all sides equal and all angles equal
are regular polygons.
Slide 9-2-7
POLYGONS
Convex
Not convex
Regular Polygons
Slide 9-2-8
CLASSIFICATION OF POLYGONS
ACCORDING TO NUMBER OF SIDES
Number of Sides
3
4
5
6
7
8
9
10
Name
Triangle
Quadrilateral
Pentagon
Hexagon
Heptagon
Octagon
Nonagon
Decagon
Slide 9-2-9
TYPES OF TRIANGLES - ANGLES
All Angles
Acute
One Right
Angle
One Obtuse
Angle
Acute Triangle
Right Triangle
Obtuse Triangle
Slide 9-2-10
TYPES OF TRIANGLES - SIDES
All Sides Equal Two Sides
Equal
No Sides Equal
Equilateral
Triangle
Scalene
Triangle
Isosceles
Triangle
Slide 9-2-11
TYPES OF QUADRILATERALS
A trapezoid is a quadrilateral with
one pair of parallel sides.
A parallelogram is a quadrilateral
with two pairs of parallel sides.
A rectangle is a parallelogram
with a right angle.
Slide 9-2-12
TYPES OF QUADRILATERALS
A square is a rectangle with all
sides having equal length.
A rhombus is a parallelogram
with all sides having equal length.
Slide 9-2-13
ANGLE SUM OF A TRIANGLE
The sum of the measures of the angles of any
triangle is 180°.
Slide 9-2-14
EXAMPLE: FINDING ANGLE MEASURES
IN A TRIANGLE
Find the measure of each angle in the triangle below.
x°
Solution
(x + 20)°
(220 – 3x)°
x + x + 20 + 220 – 3x = 180
–x + 240 = 180
x = 60
Evaluating each expression we find that the angles
are 60°, 80° and 40°.
Slide 9-2-15
EXTERIOR ANGLE MEASURE
The measure of an exterior angle of a triangle is
equal to the sum of the measures of the two
opposite interior angles.
2
4
1
3
The measure of angle 4 is equal to the sum of
the measures of angles 2 and 3. Two other
statements can be made.
Slide 9-2-16
EXAMPLE: FINDING ANGLE MEASURES
IN A TRIANGLE
Find the measure of the exterior indicated below.
(x + 20)°
(3x – 40)°
Solution
x°
x + x + 20 = 3x – 40
2x + 20 = 3x – 40
x = 60
Evaluating the expression we find that the
exterior angle is 3(60) – 40 =140°.
Slide 9-2-17
CIRCLE
A circle is a set of points in a plane, each of
which is the same distance from a fixed point
(called the center).
Slide 9-2-18
CIRCLE
A segment with an endpoint at the center and an
endpoint on the circle is called a radius (plural: radii).
A segment with endpoints on the circle is called a
chord.
A segment passing through the center, with endpoints
on the circle, is called a diameter. A diameter divides a
circle into two equal semicircles.
A line that touches a circle in only one point is called
a tangent to the circle. A line that intersects a circle
in two points is called a secant line.
Slide 9-2-19
CIRCLE
P
PR is a diameter.
O is the center
OQ is a radius.
PQ is a secant line.
Q
O
R
PQ is a chord.
PQ is an arc.
T
RT is a tangent line.
Slide 9-2-20
INSCRIBED ANGLE
Any angle inscribed in a semicircle must be
a right angle.
To be inscribed in a semicircle, the vertex of the
angle must be on the circle with the sides of the
angle going through the endpoints of the
diameter at the base of the semicircle.
Slide 9-2-21