9+10+11......+30=???

Răspuns :

30(30+1):2= 30×31:2= 930:2= 465

9 + 10 + 11 + ...... + 30 = 429

→ stabilesc cati termeni are suma

30 - 9 + 1 = 21 + 1 = 22  termeni are suma

→ aplic formula sumei lui Gauss

= 22 x ( 9 + 30 ) / 2 =

= 11 x 39 =

= 429