0
C = 2 pi r (divide by 2pi to both sides)
C/2pi = r (substitute 13 pi cm for C)
r = (13 pi cm)/(2 pi) = (13/2)(pi/pi) cm = 6.5 cm
C= pi*d
13=pi*d
13/pi=d
r=d/2
r=13/(2pi)
2 x 13 x pi = 81.68
Circumference of a circle = 2*pi*radius or pi*diameter
Circumference = 2*pi*13 = 81.68140899
pi = 3.1415926535897
2*pi*radius = circumference
2*pi*radius = 13
Making the radius the subject of the formula is:
radius = 13/(2*pi) = 2.06901426 feet
Check: 2*pi*2.06901426 = 13 feet