P

A

BC

Central Angle : An Angle whose vertex is at the center of the

circleMinor ArcMajor Arc

Less than 180°

More than 180°

ABACB

To name: use 2 letters

To name: use 3 letters

APB is a Central Angle

P

E

F

D

Semicircle: An Arc that equals 180°

EDF

To name: use 3 letters

THINGS TO KNOW AND REMEMBER ALWAYS

A circle has 360 degrees

A semicircle has 180 degrees

Vertical Angles are Equal

A

C

BD

A C

B D

measure of an arc = measure of central angle

A

B

C

Q 96m AB

m ACB

m AE

E

=

=

=

96°

264°

84°

Ð EQA is a central angle.

If Ð EQA is 80° then EA = 80°

Arc Addition Postulate

A

B

C

m ABC =

m AB + m BC

Arc Addition Postulate: The measure of an arc formed by two adjacent arcs is the sum of the measures of the two arcs.

Tell me the measure of the following arcs.

80100

40

140A

B

C

D

R

m DAB =

m BCA =

240

260

PQ has a measure of 90° in R.

Find the length of PQ

R Q

P

6P

Q

R12