Puzzle C8

Area of a Circle

Write a program that calculates the area of a circle who's radius is entered as a command line parameter.

#include <stdio.h>
#include <stdlib.h>
#include <math.h>

int main(int argc, char *argv[])
{
  if ( argc == 2 )
  {
    . . . .
  }
  return 0;
} 

Use M_PI from the math library.