求有向面积（多边形面积）

#include<stdio.h>#include<math.h>#define MAX 1001int main(){int n,i;double x[MAX],y[MAX],dsum;while(scanf("%d",&n)!=EOF){for(i=0; i<n; i++)scanf("%lf%lf",&x[i],&y[i]);x[n]=x[0];y[n]=y[0];dsum=0;for(i=0; i<n; i++)dsum+=x[i]*y[i+1]-x[i+1]*y[i];dsum=fabs(dsum)/2;printf("%.3lf/n",dsum);}return 0;}