Question

9+10+11+....30= este egal cu cat?

Answer 1

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

Answer 2

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