Bai 3:

uses crt;
var a,b,r:longint;
function dthcn(a,b:longint):longint;
begin
   dthcn:=a*b;           
end; 
function dtht(r:longint):real;
begin
   dtht:=r*r*pi;           
end;      
begin
   clrscr;
   write('Nhap chieu dai, chieu rong, ban kinh:');readln(a,b,r);
   writeln('Dien tich con lai: ',dthcn(a,b)-dtht(r):0:2);
readln
end.

Bai 4: 

uses crt;
var a,b,h:longint;   
function ttg:real;
begin
   ttg:=a*b*h/2.5;
end;
begin
   clrscr;
   write('Nhap chieu dai, chieu rong, chieu cao:');readln(a,b,h);
   writeln('Can mat ',ttg,' gio de be duoc bom day');
readln
end.

Bai 5:

uses crt;
var r,h:longint;   
function ttg:real;
begin
   ttg:=r*r*pi*h;
end;
begin
   clrscr;
   write('Nhap ban kinh, chieu cao:');readln(r,h);
   writeln('Can mat ',ttg,' gio de gieng can nuoc');
readln
end.