3 + 4 + 5 + ....... + 30 = 462
→ stabilesc cati termeni are suma cu ratia 1 ( 4 - 3 = 1; 5 - 4 = 1 .... )
30 - 3 + 1 = 27 + 1 = 28 termeni are suma
→ aplic formula sumei lui Gauss
= 28 × ( 3 + 30 ) / 2 =
= 14 × 33 =
= 462
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15 + 16 + 17 + ..... + 70 = 2 380
70 - 15 + 1 = 56 de termeni are suma
= 56 × ( 15 + 70 ) / 2 =
= 28 × 85 =
= 2 380
