C Program to Calculate Area and Circumference of Circle  Circumference of a Circle  = 2 * π * r. Here PI (π) is a Greek Letter and r is a Radius (1/2 Of Diameter ). PI (π) =  3.141592653589793  Approx. It Is A Constant Value for more about Circumference    C Program to Find Area and Circumference of Circle DOWNLOAD CODE #include <stdio.h>   #define PI 3.14159   int main(){     float radius ,aera, circumfrence;         //input radius form the user     printf("Enter the redius of the circle : ");     scanf("%f", &radius);         //calculate area of the circle     aera = PI * radius * radius ;         //calculate circumfrence of the circle     circumfrence = 2 * PI *radius;        //display result ...