#include <math.h>
#include <stdio.h>
int main(){
double x = -10;
double const dx=.1;
double s = 0;
while(x < 10) {s += exp(- x*x); x += dx;}
{double const i = s*dx;
printf("I = %20.16f, I*I = %20.16f\n", i, i*i);}
return 0;}
 
// yields I =   1.7724538509055170, I*I =   3.1415926535897967