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Circle

in a plane, the set of all points equidistant from a given point

Center

Each regular polygon has a center because it can be inscribed in a circle.

radius

From center to a point on the circle.

congruent circles

circles with congruent radii

diameter

A straight line passing through the centre of a circle to touch both sides of the circumference.

Central Angle

an angle whose vertex is the center of the circle

semicircle

half of a circle

arc

part of a circle

minor arc

smaller than a semicircle; named by its endpoint with an arc symbol over the letters - ⌒

major arc

larger than a semicircle; named by its endpoints and another point between the endpoints on the arc with an arc symbol over the letters - ⌒

adjacent arcs

arcs of the same circle that have exactly one point in common

Circumference

2πr

P⌊

∏= 22/7= 3.14...

concentric circles

coplanar circles that have the same center

arc measure

always in degree

arc length

a fraction of the circumference

congruent arcs

arcs with the same measure or length

c

∏d or 2∏r

p

ns (number of sides ⋅ side length)

a

1/2⋅a⋅p (1/2⋅ apothem⋅ perimeter)

Sector of a Circle

region bounded by an arc of the circle and the two radii to the arc's endpoints; names using one arc endpoint, the center of the circle, and the other arc endpoint

segment of a circle

part of a circle bounded by an arc and the segment joining its endpoints

Area.

The space enclosed by a shape.

Radius.

The distance from the center to the edge of a circle.

Formula to find diameter

Diameter = 2 x radius (d=2r)

Formula to find radius

Radius = diameter/2 (r=d/2)

Formula to find circumference

Circumference = 3.14 x diameter (C=3.14d)

Formula to find area

Area = 3.14 x radius squared