算法模板——计算几何1（图形面积）

 1 var
 2    i,j,k,l,m,n:longint;
 3    a:array[0..100000,1..2] of longint;
 4 function surface:extended;inline;
 5          var
 6             i,j:longint;
 7          begin
 8               j:=0;
 9               for i:=1 to n do
10                   j:=j+(a[i,1]*a[(i mod n)+1,2])-(a[(i mod n)+1,1]*a[i,2]);
11               exit(abs(j)/2);
12          end;
13 begin
14      readln(n);
15      for i:=1 to n do readln(a[i,1],a[i,2]);
16      writeln(surface:0:2);
17      readln;
18 end.
19