FUNCTION bisekt(a,b:real):real;
VAR fbeib:boolean; x: real;
BEGIN
  fbeib:=f(b)>0; x := (a+b)/2;
  WHILE (a<x) AND (x<b) DO
  BEGIN
  IF fbeib=(f(x)>0) THEN b := x ELSE a := x ;
    x := (a+b)/2;
  END;
  bisekt:=x
END;