Circle illustration showing a radius, a diameter, the centre and
the circumference
A circle is a simple shape of Euclidean
geometry consisting of the set of points in a plane that is a given distance from a given
point, the centre. The distance between any of the points and the centre is
called the radius.
Terminology
A circle's diameter is the length of a line segment whose endpoints lie on the circle and which passes
through the centre. This is the largest distance between any two points on the
circle. The diameter of a circle is twice the radius,
or distance from the centre to the circle's boundary. The terms
"diameter" and "radius" also refer to the line segments
which fit these descriptions. The circumference is the distance around the outside of
a circle.
A chord is a line segment whose endpoints lie
on the circle. A diameter is the longest chord in a circle. A tangent to a circle is a straight line that
touches the circle at a single point, while a secant is an extended chord: a straight line
cutting the circle at two points.
An arc of a circle is any connected part of the circle's circumference. A sector is a region bounded by two radii and
an arc lying between the radii and a segment is a region bounded by a chord and an
arc lying between the chord's end points.
Chord, secant, tangent, and diameter.


http://en.wikipedia.org/wiki/Circle
The
above explanation is copied from Wikipedia, the free encyclopedia and is
remixed as allowed under the Creative Commons Attribution ShareAlike 3.0 Unported
License.
(Our solved example in mathguru.com uses the below concept. This
is our own explanation, it is not taken from Wikipedia.)
·
Angles in the same segment of a circle
are equal.
· The sum of all the angles in a Triangle is 180(Angle Sum
Property)