Folosim suma Gauss , dar intai adaugam cifrele de la 1 la 8 si dupa le scadem
1+2+...+7+8+9+...+30-(1+2+...+8)=30(30+1)/3-8(8+1)/2=15•31-4•9=465-36=429
9 + 10 + 11 + ..... + 30 = 429
→ stabilesc cati termeni are suma, cu ratia 1
30 - 9 + 1 = 22 termeni are suma
→ aplic formula sumei lui Gauss
= 22 × ( 9 + 30 ) / 2 =
= 11 × 39 =
= 429
