Ta có
a = 2002 x 2002 = (2000 + 2) x 2002 = 2000 x 2002 + 2 x 2002 = 2000 x 2002 + 4004
b = 2000 x 2004 = 2000 x (2002 + 2) = 2000 x 2002 + 2000 x 2 = 2000 x 2002 +4000
Mà 2000 x 2002 + 4004 > 2000 x 2002 +4000 (vì 2000 x 2002 = 2000 x 2002 ; 4004 > 4000)
Vậy a > b