% cat calculus.c

/* calculus - Estimate the area under the curve Y=X**2 from 0-1

*  Written by: C. Severance - Tue Dec  7 17:15:12 EST 1993

*/

#include <math.h>

main() {

  float x,area,rect,error;
  double tmp;
  double fabs();

  area = 0.0;
  for( x=0.0; x<=1.00; x=x+0.1) {
    rect = 0.1 * (x * x);
    area = area + rect;
  }

/* Print out the approximate area and actual area and the error */

  printf("approximate area = %f\n",area);
  printf("actual area = %f\n",1.0/3.0);
  tmp =  (1.0/3.0) - area;
  error = fabs ( tmp ) ;
  printf("error = %f\n",error);
}
% cc calculus.c
% a.out
approximate area = 0.285000
actual area = 0.333333
error = 0.048333
%