Download Lesson 32

CIRCLES AND
CIRCUMFERENCE
Measuring Angles and Arcs
Circles
■ A circle is a locus or set of all points in a plane equidistant
from a given point called the center of the circle.
Special Segments in a Circle
■ Radius – a segment with endpoints at the center and on the
circle.
■ Chord – a segment with endpoints on the circle.
■ Diameter – a chord that passes through the center and is
made up of collinear radii.
Special Segments in a Circle
Circle Pairs
■ Two circles are congruent if and only if they have congruent
radii.
■ All circles are similar.
■ Concentric circles are coplanar circles that have the same
center.
Circle Pairs
■ Two circles can intersect in two different ways.
Circumference
■ The circumference of a circle is the distance around the
circle. If a circle has a diameter d or radius r, the
circumference C equals the diameter times pi or twice the
radius times pi.
‘Scribes
■ A polygon is inscribed in a circle if all of its vertices lie on the
circle.
■ A circle is circumscribed about a polygon if it contains all the
vertices of the polygon.
Measuring Angles and Arcs
■ A central angle of a circle is an angle with a vertex in the
center of the circle. Its sides contain two radii of the circle.
Sum of Central Angles
■ The sum of the measures of the central angles of a circle with
no interior points in common is 360.
Examples
Examples
Arc
■ An arc is a portion of a circle defined by two endpoints. A
central angle separates the circle into two arcs with measures
related to the measure of the central angle.
Arcs and Arc Measure
Congruent Arcs
■ In the same circle or in congruent circles, two minor arcs are
congruent if and only if their central angles are congruent.
Arc Addition Postulate
■ The measure of an arc formed by two adjacent arcs is the
sum of the measures of the two arcs.
Arc Length
■ The ratio of the length of an arc l to the circumference of the
circle is equal to the ratio of the degree measure of the arc to
360.
Examples
■ Find the length of arc AB. Round to the nearest hundreth.
Examples
■ Find the length of arc AB. Round to the nearest hundreth.
Radian Measure
■ The radian measure of a central angle is the ratio of the arc
length to the radius of the circle:
Examples
■ Find the length of arc ZY. Round to the nearest hundreth.
Examples
■ Find the length of arc ZY. Round to the nearest hundreth.