a = 1 + 2 + 3 + ...... + 11
a = 11 × ( 1 + 11 ) / 2
a = 11 ×12 / 2
a = 66
___________________
b = 9 + 10 + 11 + ..... + 18
→ stabilesc cati termeni are suma, cu ratia 1 ( 10 - 9 = 1; 11 - 10 = 1,...... )
18 - 9 + 1 = 10 termeni are suma
→ aplic formula sumei lui Gauss
b = 10 × ( 9 + 18 ) / 2
b = 10 × 27 / 2
b = 270 / 2
b = 135
___________
a + b = 66 + 135 = 201
