#include<bits/stdc++.h>
using namespace std;
#define d double
int main()
{
    d n;cin>>n;
    cout<<fixed<<setprecision(4)<<n*2<<" "<<fixed<<setprecision(4)<<2*n*3.14159<<" "<<fixed<<setprecision(4)<<3.14159*n*n<<endl;
    return 0;
}

《printf 最好用的一集》

#include <bits/stdc++.h>
using namespace std;
double r;
int main() {
	cin >> r;
	printf("%.4f %.4f %.4f", r * 2, r * 2 * 3.14159, r * r * 3.14159);
	return 0;
}

#include <bits/stdc++.h> using namespace std; double r, c, s, d; int main() { cin >> r; d = r * 2, c = d * 3.14159, s = r * r * 3.14159; cout << fixed << setprecision(4) << d << " " << c << " " << s << "\n"; return 0; }