Explanation:
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.
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.
Length of circumference
The ratio of a circle's circumference to its diameter is π (pi), an irrational constant approximately equal to 3.141592654.
Thus the length of the circumference C is related to the radius rand diameter d by:
(Our solved example in mathguru.com uses this concept)
http://en.wikipedia.org/wiki/Circle
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