好像知道了
变量都声明在循环外面就可以。
#include<stdio.h>
const double PI=3.141592654;
int main()
{
    //freopen("input.txt","r",stdin);
    int n;
    int i=1;
    double Area;
    int year;
    double x,y;
    scanf("%d",&n);
    for(i=1;i<=n;i++)
    {

        scanf("%lf%lf",&x,&y);

        Area=0.5*PI*(x*x+y*y);

        year=(int)(Area/50.0+1.0);
        printf("Property %d: This property will begin eroding in year %d.",i,year);
        printf("\n");

    }
    printf("END OF OUTPUT.");
    return 0;
}