A circle is the path traces out by point which moves in a plane so that it is always the same distance from a fixed point in that plane.
The fixed point is called the center of the circle and the distance of the point from the center of the circle is called the radius.
If the point could move anywhere in space and where not obliged to lie in a plane, it would trace out a sphere.

Diameter: A diameter of a circle is a line through its center terminated at  each end by the circumference of the circle.  chord: A chord of a circle is a line joining any two points  on the circle.circumference: The circumference of a circle is the distance round the circle .Arc: An arc of a circle is a circle is part of the circumference. A major arc is an arc greater then half the circumference. A minor arc is an arc less than half the circumference.

C = 2`pi` r=2`pi xx`5 cm=10`pi`cm
                                        =31.42 cm
                                        =31.4 cm  (to 3 sig. fig)

C = 2`pi`r 

`:.` r = `C/(2pi)`

         = `44/(2 xx 22/7)`

         = 7.

The radius of the circle is 7 cm.

A = `pi r^2`

   = `22/7 xx (7)^2 cm^2`

   = 22 `xx 7 cm^2`

   = 154 `cm^2`