分数序列的前n项之和

分数序列 2/1,3/2,5/3,8/5,13/8,21/13,.... 求这个分数序列的前n项之和。

#include<iostream>
#include<stdio.h>
using namespace std;
int main()
{   int N;
    double a = 2;
    double b = 1;
    double c = 0;
    double temp;
    double sum = 0;
    int count;
    cin >> N;
    for (int i = 0; i < N;i++)
    {
        cin >>count;
        for (int j = 0; j < count;j++)
        {
            c = a / b;
            sum += c;
            temp = a;
            a = a + b;
            b = temp;
        }
        printf("%.4f\n", sum);
        a = 2;
        b = 1;
        sum = 0;
    }
    
    system("pause");
    return 0;
}

#include <iostream>
#include <vector>
#include <stdio.h>
using namespace std;

double sum(int n)
{
    double a1=2,a2=3,b1=1,b2=2,temp1,temp2;
    double result=0;
    if(n==1)
        return a1/b1;
    if(n==2)
        return a1/b1+a2/b2;
    if(n>2)
    {
        for(int i=3;i<=n;i++)
        {
            temp1=a2;
            a2=a1+a2;
            a1=temp1;
            temp2=b2;
            b2=b1+b2;
            b1=temp2;
            result+=a2/b2;
        }
    }
    return result+3.5;
}

int main()
{
    int i,k,n;
    vector<double> a;
    cin>>k;
    for(i=0;i<k;i++)
    {
        cin>>n;
        double x=sum(n);
        a.push_back(x);
    }
    for(int j=0;j<k;j++)
        printf("%.4f\n",a[j]);
    return 0;
}