a = 1 + 2 + 3 + ....... + 11
a = 11 × ( 1 + 11 ) / 2
a = 11 × 12 / 2
a = 11 × 6
a = 66
___________________
b = 9 + 10 + 11 + ....... + 19
→ stabilesc cati termeni are suma cu ratia 1
19 - 9 + 1 = 11 termeni are suma
→ aplic formula sumei lui Gauss
b = 11 × ( 9 + 19 ) / 2
b = 11 × 28 / 2
b = 11 × 14
b = 154
___________
a + b = 66 + 154 = 220 → suma numerelor a si b
