1 + 2 + 3 + .... + 16 = 16 × ( 1 + 16 ) / 2 = 8 × 17 = 136
______________________________________
1 + 2 + 3 + ..... + 100 = 5 050
= 100 × ( 1 + 100 ) / 2 =
= 50 × 101 =
= 5 050
_______________________
S = 21 + 22 + ..... + 100
→ stabilesc cati termeni are suma, cu ratia 1
100 - 21 + 1 = 79 + 1 = 80 termeni are suma
→ aplic formula sumei lui Gauss
S = 80 × ( 21 + 100 ) / 2
S = 40 × 121
S = 4 840
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17 + 18 + .... + 40 = 684
= ( 40 - 17 + 1 ) termeni × ( 17 + 40 ) / 2 =
= 24 × 57 / 2 =
= 12 × 57 =
= 684