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
vì 2000 x 2002 = 2000 x 2002
4004>4000
suy ra 2000x 2002 +4004 > 2000 x 2002 +4000
a>b
vậy a>b