Ta có:
A = 1987657 x 1987655
= ( 1987656 + 1 ) x ( 1987655 )
= 1987656 x 1987655 + 1987655
B = 1987656 x 1987656
= 1987656 x ( 1987655 + 1 )
= 1987656 x 1987655 + 1987656
Vì 1987656 x 1987655 + 1987655 < 1987656 x 1987655 + 1987656 nên
A < B
Vậy A < B
A = 1987657 x 1987655
A = ( 1987656 + 1 ) x 1987655
A = 1987656 x 1987655 + 1987655
B = 1987656 x 1987656
B = ( 1987655 + 1 ) x 1987656
B = 1987655 x 1987656 + 1987656
Vậy B > A
A = 1987657 x 1987655
A = ( 1987656 + 1 ) x 1987655
A = 1987656 x 1987655 + 1 x 1987655
B = 1987656 x 1987656
B = ( 1987655 + 1 ) x 1987656
B = 1987655 x 1987656 + 1 x 1987656
1987656 x 1987655 = 1987655 x 1987656; Mà 1 x 1987655 < 1 x 1987656
=> A < B