1+2+3+.......+50 = (1+50)·50:2 = 51·25 = 1275
50 = numarul de termeni
1+3+5+....+39 = (1+39)·20:2 = 40·10=400
20 = numarul de termeni = (39-1):2 +1 = 19+1 = 20
1+2+3+....+39 = (1+39)·39:2 = 20·39 = 780
39 = numarul de termeni
4+5+6+....+99 = (4+99)·96:2 = 103·48 = 4944
99 - 3 (lipsesc 1,2,3) = 96 termeni
