PI的求法

1671年(J.Gregory)

π/4=1-1/3+1/5-1/7+1/9-1/11+....

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

int main()
{
    double pi = 0.0;
    int flag, i;
    int n;

    while(~scanf("%d", &n))
    {
        pi = 1.0;
        flag = 0;
        for( i = 3; i < n; i += 2 )
        {
            pi += (flag == 0 ? -1.0 : 1.0)/i;
            flag = !flag;
        }

        printf("%lf\n", 4.0*pi);
    }

    return 0;
}

欧拉(Leonhard Euler)

π^2/8=1+1/3^2+1/5^2+1/7^2+.....

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

int main()
{
    double pi = 0.0;
    int n, i;

    while(~scanf("%d", &n))
    {
        pi = 1.0;
        for( i = 3; i <= n; i += 2 )
        {
            pi += 1.0/(i*i);
        }
        pi *= 8;
        pi = sqrt( pi );
        printf("%lf\n", pi);
    }

    return 0;
}