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#include <iostream>#include <math.h>using namespace std;const double pi=acos(-1);int main(){ double r; cin>>r; printf("%.4f ",2*r); printf("%.4f ",pi*r*2); printf("%.4f",pi*r*r);}

#include <iomanip>float s;cout<<setiosflags(ios::fixed)<<setprecision(2)<<s;

头文件stdio.h用scanf不必用cin#include<stdio.h>int main(){int n;double m;printf("please input:\n");scanf("%d%lf",&n,&m);printf("n=%d, m=%lf \n",n,m);scanf("%d,%lf",&n,&m);printf("n=%d, m=%lf \n",n,m);}解答：lf：long float——用于表示小

头文件stdio.h用printf不必用cout例：#include<stdio.h>int main(){ printf("Hello,World!\n"); return 0;}其中\n用来换行例：#include<stdio.h>int main(){ printf("Hello,World!\nHello,China!");}双...