实验3-1

#include<stdio.h>
int main()
{
    double r,s;              /*定义两个变量*/

    printf("Enter r:");
    scanf("%Lf",&r);

    if(r<=0)
    {
        printf("Sorry! it is wrong!");
    }
    else
    {
        s=3.1415926*r*r;
        printf("s=%.2f\n",s);
    }
    
    return 0;
}