The area of a circle is pi x radius^2
The radius is 5 (half of the diameter)
So pi x 25 is 25pi^2.
Pi is the ratio of the diameter to the circumference
circumference/diameter=pi
that's why c=pid or c=2pir
some digits are <span>3.14159265358979323846264338327950288419716939
937510582097494...</span>