The word is pi, not pie. It is the ratio of the radius of a circle to its circumference. Circumference is C, D is the diameter, which equals 2 x the radius (R)

It is equal to 22/7, or 3.14159......

Now, C (circumference) = piD or C = 2piR

It is equal to 22/7, or 3.14159......

Now, C (circumference) = piD or C = 2piR