#define PI 3.14159
#define AREA(r) PI * r * r
#define CIRC(r) 2 * PI * r

Circle with radius 5:
  Area      = AREA(5)
  Circumference = CIRC(5)

Circle with radius 10:
  Area      = AREA(10)
  Circumference = CIRC(10)

#define MAX(a,b) (a >= b ? a : b)
Maximum of 10 and 7 is MAX(10, 7)
Maximum of 3 and 9 is MAX(3, 9)