/* 
 *         　　台形則による定積分
 */
#include <math.h>
#include <stdio.h>

int main(void)
{
  int    j;
  int    ntrial;
  double x0 = 0.0, x, y;
  double s = 0.0;
  double h;
  double sreal, serror;

  printf("Input: Number of trials:\n");
  scanf("%u", &ntrial);
  h=1.0/ntrial;

  j = 0;
  x = x0;
  y = exp(-x);
  s = s + 0.5*y;

  for (j = 1; j <= ntrial-1; j ++) {
    x = x + h;
    y = exp(-x);
    s = s + y;
  }

  j = j +1;
  x = x + h;
  y = exp(-x);
  s = s + 0.5*y;

  s = s / ntrial;
  sreal=1.0-exp(-1.0);
  serror=s-sreal;
  printf("s = %1.10f \n", s);
  printf("sreal = %1.10f \n", sreal);
  printf("serror = %1.10f \n", serror);
}