$\text{Đáp án+giải thích các bước giải:}$
$\text{a) Ta có : A = 53 × 517}$
$\text{= ( 53 + 4 ) × 517 - 4 × 517}$
$\text{= 57 × 517 - 4 × 517}$
$\text{= 57 × ( 517 - 40 ) + 40 × 57 - 4 × 517}$
$\text{= 57 × 531 - 4}$
$\text{B = 57 × 531}$
$\text{⇒ A < B}$
$\text{b) Ta có :A = 25 × 26261}$
$\text{= ( 25 + 1 ) × 26261 - 26261}$
$\text{= 26 × 26261 - 26261}$
$\text{= 26 × ( 25251 + 1010 ) - 26261}$
$\text{= 26 × 25251 + 26 × 1010 - 26261}$
$\text{= 26 × 25251 + ( -1 )}$
$\text{B = 26 × 25251}$
$\text{⇒ A < B}$
