1 + 3 + 5 + ......+ 21 = 121
→ aflu cati termeni are suma: ( 21 - 1 ) : 2 + 1 = 20 : 2 + 1 = 10 + 1 = 11 termeni, ratia = 2 ( 3 - 1 = 2; 5 - 3 = 2 ....)
→ suma primelor 11 numere naturale impare
→ aplic formula sumei lui Gauss
= 11 × ( 1 + 21 ) / 2 = 11 × 22 / 2 = 11 × 11 = 121
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11 + 13 + 15 + ......+ 1 081 =
( 1 081 - 11 ) : 2 + 1 = 1 070 : 2 + 1 = 536 → termeni are suma
= 536 × ( 11 + 1 081 ) / 2 = 268 × 1 092 =
= 292 656
