1 997 + 2 × ( 1 + 2 + 3 + ...... + 1 996 ) = 3 988 009
= 1 997 + 2 × [ 1 996 × ( 1 + 1 996 ) ] / 2 =
= 1 997 + 1 996 × 1 997 =
= 1 997 × ( 1 + 1 996 ) =
= 1 997 × 1 997 =
= 1 997² =
= 3 988 009
