2002 x 20022001 - 2001 x 20022002=
2002 x 20022001 - 2001 x 10001 x 2002=
20020000
A = 2002 x 20022001 - 2001 x 20022002
= (2001+1) x 20022001 - 2001 x (20022001+1)
= 2001 x 20022001 + 1 x 20022001 - 2001 x 20022001 - 2001 x 1
= 2001 x 20022001 + 20022001 - 2001 x 20022001 - 2001
= 2001 x 20022002 - 2001 x 20022001 + 20022001 - 2001
= 2001 x (20022002 - 20022001) + 20022002 - 2001
= 2001 x 1 + 20022002 - 2001
= 2001 + 20022002 - 2001
= 2001-2001+20022002
= 0+20022002
= 20022002
như này mới đúng!