Brian McCall

**Univ. of Wisconsin**

J.D. Univ. of Wisconsin Law school

Brian was a geometry teacher through the Teach for America program and started the geometry program at his school

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A chord is a line segment whose endpoints are on a circle. A diameter is a chord that passes through the center of a circle. Another one of the **parts of a circle** is a radius, which is a line segment with one endpoint at the center and one endpoint on the circle. Congruent circles have congruent radii (the plural of radius). Concentric circles have the same center. A central angle has a vertex on the center and endpoints on the circle.

The definition of a circle is the set

of all points in the same plane

that are a given distance from a

given point.

So here we have a

given point, our center, and the circle,

which is the black line. And

those are all the points that are

a given distance away from the

center. And that given distance

is your radius. If you have

a chord or a segment that passes through

the center, that is called a

diameter.

Now, two commonly confused

terms regarding circles are

congruent circles and concentric circles.

Circles whose radii have

the same measure are congruent. Circles

who share the same center

are concentric, which means the center

of the smaller circle is the same

center as the larger circle. The radiis

are different, so they are not

going to be congruent.

However, you

could have concentric and congruent

circles in which case I could say that

here I've drawn two concentric

and congruent circles because they are

just going to overlap because they

have the same center and the same radius.

So keep that in mind when

you are answering questions, usually

true and false and matching, about

circles.