真的是完完全全的菜鸟 就解

少分号
double

pai

弄出来了
#include <stdio.h>
#include <stdlib.h>
/* run this program using the console pauser or add your own getch, system("pause") or input loop */
int main(int argc, char *argv[]) {
double r,a,b,pai=3.1415926;
scanf ("%lf",&r);
a = 2*r*pai;
b = pai*r*r;
printf ("%lf,%lf",a,b);
}