1001²= ( 1 000 + 1 )² =
= 1 000² + 2 × 1000 × 1 + 1²
=1 000 000 + 2 000 + 1
= 1 002 001
999² = ( 1 000 – 1 )² = 1 000² – 2 × 1000 × 1 + 1²
=1 000 000 – 2 000 + 1 = 998 001
1001 × 999 = ( 1 000 + 1 ) ( 1000 – 1 ) = 1 000² – 1² =1 000 000 – 1 = 999 999
