I'm sorry....
В условии сказано "Гарантируется, что точки (x1, y1) и (x2, y2) не лежат на прямой, проходящей через точки (xm1, ym1) и (xm2, ym2)" Это значит что они одновременно не лежат или одна может лежать а другая нет?...

Edited by author 21.01.2006 13:51

It means that each of these points doesn't belong to the line, passing through the points (xm1, ym1) and (xm2, ym2).

Then i could not understand how this solution gets WA#23
Const
 Eps = 1e-15;
function Eq(a, b : Extended) : boolean;
begin
  Result := Abs(a-b) <= eps;
end;

Function Less(a, b : Extended): boolean;
begin
  Less := Eq(a,b) or ((a - b) <= -eps);
end;

Function Less1(a, b : Extended): boolean;
begin
  Less1 := ((a - b) <= -eps);
end;


Var
 x1,x2,x3,x4,x5,y1,y2,y3,y4,y5 : extended;
 A,B,C,A1,b1,c1 : extended;
BEGIN
 Read(x1,y1);
 Read(x2,y2);
 Read(x3,y3);
 Read(x4,y4);
 x5 := 2*(y3-y4)*((x3-x4)*(y2-y3) - (x2-x3)*(y3-y4))/
       ( (x3-x4)*(x3-x4) + (y3-y4)*(y3-y4)) + x2;
 y5 := 2*(x4-x3)*((x3-x4)*(y2-y3) - (x2-x3)*(y3-y4))/
       ( (x3-x4)*(x3-x4) + (y3-y4)*(y3-y4)) + y2;
 A := y1-y5;
 B := x5-x1;
 C := -(A*x1+B*y1);
 A1 := y3-y4;
 B1 := x4-y3;
 C1 := -(A1*x3 + B1*y3);
 if (Less((A*x4+B*y4+C)*(A*x3+B*y3+C),0)) and ( Less1((a1*x1+b1*y1+c1)*(a1*x5+b1*y5+c1),0)) then
  Writeln('VISIBLE')
 Else
  Writeln('INVISIBLE');
END.

Edited by author 21.01.2006 19:32