2)
1 + 3 + 5 + ......+ (2x -1) = 100
aflam cati termeni are S = > (2x - 1 + 1)/2 = 2x/2 = x (termeni)
(1 + 2x - 1) × x / 2 = 100
2x × x / 2 = 100
x² = 100
x = √100 = 10
2x - 1 = 2 × 10 - 1 = 19 ( ultimul termen al sirului )
1 + 3 + 5 + ....+ 19 = 100
