1 + 2 + 3 +... + 100 =
100 × (100 + 1) : 2 =
100 × 101 : 2 =
50 × 101 =
5050
51 + 52 + ... + 102 =
(1 + 2 + 3 +...+ 102 ) - (1 + 2 + 3 +...+ 50) =
102 × 103 : 2 - 50 × 51 : 2 =
51 × 103 - 51 × 25 =
51 × (103 - 15) =
51 × 78 =
3 978
5 050 - 3 978 = 1 072
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