注意

• 輸入值是浮點(diǎn)數(shù)
• 判斷最小一項(xiàng)是否小于等于輸入值
• 直接計(jì)算會超出長整形
• 保留6位小數(shù)
• 輸入時double類型需要用lf來讀入

代碼

#include <iostream>
#include <cstdio>
using namespace std;

int main(){

    double x;
    scanf("%lf", &x);
    double sum = 1;
    int t = 1;
    double last = 1;
    while(last >= x){
        last = last*t/(2*t+1);
        sum += last;
        t++;
    }
    printf("%.6f\n", sum*2);
    return 0;
}