uses crt;
var i,n: integer;
s: real;
begin
clrscr;
write ('nhap n: ');  readln (n);
for i:=1 to n do
if i mod 2<>0 then s:=s+(1/i)
else s:=s-(1/i);
writeln ('a/ tong la: ',s:1:2);
s:=0;
for i:=1 to n do
s:=s+(i*(i+1))/((i+2)*(i+3));
writeln ('b/ tong la: ',s:1:2);
readln;
end.

a)

program bai_giai;
uses crt;
var n,i:integer;
    S:real;
begin
    clrscr;
    write('Nhap n: ');  readln(n);
    S:=0;
    for i:=1 to n do
        if i mod 2=1 then S:=S+1/i else S:=S-1/i;
    write('S = ',s:0:2);
    readln;
end.

b)

program bai_giai;
uses crt;
var n,i:integer;
    S:real;
begin
    repeat
         clrscr;
         write('Nhap n: ');  readln(n);
         if (n<=5) or (n>100) then
             begin
                 write('5 < n <= 100. Nhap lai!');
                 readln;
             end;
    until (n>5) and (n<=100);
    S:=0;
    for i:=1 to n do
        S:=S+(n*(n+1))/((n+2)*(n+3));
    write('S = ',S:0:3);
    readln;
end.