题解 | #牛牛学数列3#

思路就是暴力枚举每一项

#include<iostream>
using namespace std;
double fun(int x){
    int i,j;
    double sum=0;
    for(i=1,j=1;i<=x;i++,j+=2){
        if(i%2) sum+=j;
        else sum-=j;
    }
    return sum;
}
int main()
{
    int n;
    double ans=0;
    cin>>n;
    for(int i=1;i<=n;i++)
        ans=ans+1.0/fun(i);
    printf("%.3lf",ans);
    return 0;
}