#include<bits/stdc++.h> 
using namespace std; 
double r,s,c,d;//定义变量 
const double PI=3.14159;//定义圆周率，我写到五位小数 
int main(){ 
    cin>>r;//输入半径 
    d=r*2;//计算直径 
    c=PI*d;//计算周长 
    s=r*r*PI;//计算面积 
    cout<<fixed<<setprecision(4)<<d<<" " <<c<<" "<<s;//输出 
    return 0; 
}

挺简单的😄加油

#include <bits/stdc++.h> 
using namespace std;
#define ll long long
#define s string
#define co cout
#define ci cin
#define re return
double r,p=3.14159;
int main(){
    ios::sync_with_stdio(0);
    cin.tie(0);
    cout.tie(0);
    cin>>r;
    cout<<fixed<<setprecision(4)<<r*2;
    cout<<" ";
    cout<<fixed<<setprecision(4)<<r*2*p;
    cout<<" ";
    cout<<fixed<<setprecision(4)<<r*r*p;
    return 0;
}