Circumference is ,yes its 2Pi*r ,where r is the radius
and Radius by definition is the half of diameter(the largest chord that can fit inside a circle)
another definition shall be,radius is the distance of any point on the circimference to the center of the circle.
If diameter(D) is given then radius is half of it ,i.e r =D/2
If what given is the circumference(C) of the circle then radius is what u get by dividing it by 2Pi ,i.e r =C/(2*pi)
If area (A) of the circle is given then radius is the square root of (Area/[2*Pi]) ,i.e r = sq.rt( A/ [2*Pi] )
Dear,there is an interesting point about radius, Every radii of a circle will have one common point(Center/Orgin of the circle) and will never have two common points in a circle.
And another thing is that
if u hav a circle's circumference less than 6.29 ,then its radius will be less than 1
if circumference is less than 12.57,then radius will be less than 2
circumference < 18.85 ,then radius < 3
circumference < 25.14 ,then radius < 4
circumference < 31.43 ,then radius < 5
remember the multiples of 6 ,then u can make a guess of the range of radius.for eg a circumference having a value around 60 will have a radius value around 10 (exactly its 9.55)
similarly radius-area, area-circumference values can also be gussed with practice....