MIk làm bài 4 thôi nha 

4.

program Tin_hoc;

uses crt;

var i:integer;

     s,s2:string;

begin

clrscr;

writrln('Nhap xau S:');readln(s);

s2:='em';

for i:= 1 to length(s) do if (s[i] = 'a') and (s[i+1] = 'n') and (s[i+2] = 'h') then

begin

delete(s,i,3);

insert(s2,s,i)

end;

writeln(s2);

readln;

end.

____S: String;

____ch: char;

____i: integer;

begin

____clrscr; {lenh xoa man hinh}

____write(‘Nhap xau S: ’);

____readln(S);

       _for ch:= ‘A’ to ‘Z’ do

_______Dem[ch]:= 0;

      _ for i:=1 to length(S) do

         begin

            if S[i] in ([‘a’..‘z’]) then {co the thay bang if (S[i] >= ‘a’) and (S[i] <= ‘z’) then}

               begin

_________ch:=upcase(S[i]);

_________Dem[ch]:= Dem[ch]+1;

               end;

          end;

         {for ch:= ‘A’ to ‘Z’ do

                    write(‘So lan xuat hien cua ’,ch, ‘ trong xau la ’, Dem[ch]);}

____writeln(‘So lan xuat hien cua cac chu cai trong xau la: ‘);

           for ch:= ‘A’ to ‘Z’ do

_______write(ch:3);

____writeln;

          for ch:= ‘A’ to ‘Z’ do

_______write(Dem[ch]:3);

____readln

hết rôi nhá.

đủ luôn.