Radius of a Circle

The radius name comes from the Latin and its meaning is ray. 

Definition:              

Radius 

The distance from center of a circle to any point on the circle, or a line from center to any point on the circumference of a circle is known as “Radius”, of  a circle. 

The plural of radius is radii. All radii in a circle has same length.

Infinite radii can be drawn for any circle.

OA, OB, OC, OD, OE, OF, OG, OH, OI, OJ, OK, OL, OM, ON, OO, OP, are the radii of circle O.

The length of the complete circle is called 
circumference. 

In figure AO is radius of circle. It is half of diameter.

The longest distance that has the end points on the circle and passes through the center is called “Diameter”. 

It is a special chord(longest chord) and it is twice of radius.

                    Formula for radius

Radius = Diameter/2

r = D/2,

Where r is radius, and D is diameter.

Radius = Circumference/2𝜋  

Area = 𝜋(Radius)²

Area = 𝜋r²

C = 2𝜋r,  Where C is the Circumference, and r is the radius of the circle.
Circumference = 𝜋(2r)

Circumference = 𝜋d, (2 x radius = d) 

The value of 𝜋 is 3.14159 or 22/7.