/*  trarule.c  [Trapezoidal Rule] */

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

double func(double x)
{
    double y;

    y = 2.0 / ( 1.0 + x * x );

    return y;
}

int main(void)
{
    int i,n;
    double x[1000],y[1000];
    double x1,x2,dx,a;

    n=40;

    x1 = -1.0;
    x2 =  1.0;

    for (i=0; i<=n; i++)  {
        x[i] = x1+(x2-x1)*i/n;
        y[i] = func(x[i]);
    }

    dx = x[1]-x[0];

    a = 0.5 * (y[0]+y[n]);
    for (i=1; i<n; i++)  {
        a = a + y[i];
    }
    a = a * dx;

    printf("Sekibunchi = %f\n",a);

    return 0;
}