Type
 Arr=Array[0..3] of Real;

Var
 Xi,Yi:Arr;
 a,b,D,X,Y,Step:Real;
 i,j:Byte;
 f:Text;

Function Fun(z:real):Real;
begin
 Fun:=Ln((1+z)/(1-z));
end;

Function Lagr(NL:Byte; XL,YL:Arr; ZL:Real):Real;
var
 S,T:Real;
 k,p:Byte;
begin
 S:=0;
 for k:=0 to NL do
  begin
   T:=1;
   for p:=0 to NL do
    if p<>k then T:=T*(ZL-XL[p])/(XL[k]-XL[p]);
   S:=S+YL[k]*T;
  end;
 Lagr:=S;
end;

Begin
 Writeln(' Attention! -1<a,b<+1; a<b');
 Repeat
  Write(' a = ');
  Readln(a);
  Write(' b = ');
  Readln(b);
 Until (a>-1) and (a<1) and (b>-1) and (b<1) and (a<b);
 D:=(b-a)/3;
 {Formation of Table}
 Writeln(' Number of point     Xi              Yi');
 for i:=0 to 3 do
  begin
   Xi[i]:=a+D*i;
   Yi[i]:=Fun(Xi[i]);
   Writeln(i:8,Xi[i]:16:3,Yi[i]:16:3);
  end;
 Writeln;
 {Calculation of Lagrange interpolation}
 Write(' Step (Step<(b-a))! = ');
 Readln(Step);
 Assign(f,'Result.txt');
 Rewrite(f);
 Writeln('     X       Lagrange interpolation      Accurate value');
 Writeln(f,'     X       Lagrange interpolation      Accurate value');
 X:=a;
 Repeat
  Writeln(X:8:3,Lagr(3,Xi,Yi,X):18:3,Fun(X):26:3);
  Writeln(f,X:8:3,Lagr(3,Xi,Yi,X):18:3,Fun(X):26:3);
  X:=X+Step;
 Until X>b;
 Close(f);
 Readln
End.