HDU 2036 改革春风吹满地

一个多边形公式

s = ( x1*y2-x2*y1 ) + ( x2*y3 - x3*y2 )……( xn-1*yn - xn*yn-1 ) + ( xn*y0 - x0*yn );

一个公式就OK了

#include<stdio.h>
int n;
double x[105],y[105],sum ;
int main( )
{
    while( scanf( "%d",&n ),n )
    {
           sum = 0;
           for( int i = 0 ; i < n; ++i )
           {
                scanf( "%lf%lf",&x[i],&y[i] );
                if( i )
                    sum += x[i-1]*y[i] - x[i]*y[i-1];
            }
            sum += x[n-1]*y[0] - x[0]*y[n-1];
           printf(  "%.1lf\n",sum/2 );
           }
    return 0;
}