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Parts of a Circle - Concept 13,221 views

Teacher/Instructor Brian McCall
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

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.

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