// one4.cpp : 定義控制臺應用程序的入口點。
// 2008.10.10 
// raosu
// 

#include "stdafx.h"
#include <iostream>
using namespace std;

double m_sum1=0.0,m_sum2=0.0,m_sum3=0.0,m_N;
//decrease
double fun1(double m_N)
{	
	for(double i=m_N;i>=2;i--)
	{
		m_sum1+=1/(i*i-1);
	}
	return m_sum1;
}
//increase
double fun2(double m_N)
{	
	for(double i=2;i<=m_N;i++)
	{
		m_sum2+=1/(i*i-1);
	}
	return m_sum2;
}
//precise
double fun3(double m_N)
{
    m_sum3=1.0/2.0*(3.0/2.0-1/m_N-1.0/(m_N+1));
	return m_sum3;
}
int _tmain(int argc, _TCHAR* argv[])
{
	double N;
	cout<<"please iuput :N"<<endl;
	cin>>N;
	printf("sum1=%0.16f\n",fun1(N));
	printf("sum2=%0.16f\n",fun2(N));
	printf("sum3=%0.16f\n",fun3(N));
	getchar();	
	getchar();
	return 0;
}