Explanation:
area of a circle
A = pi × r²
circumference of a circle
C = 2 × pi × r
r = diameter / 2 (the radius is half of the diameter)
circle A
A = 63.585 = pi × 4.5² = pi × 20.25
pi = 63.585 / 20.25 = 3.14
C = 28.26 = 2 × pi × 4.5
pi = 28.26 / (2 × 4.5) = 28.26 / 9 = 3.14
circle B
A = 19.625 = pi × 2.5² = pi × 6.25
pi = 19.625 / 6.25 = 3.14
C = 15.7 = 2 × pi × 2.5
pi = 15.7 / (2 × 2.5) = 15.7 / 5 = 3.14
pi is always the same for every circle of any size. it is a natural constant defining the circumference and area of every circle in relation to the actual radius or diameter.