La toate folosești formula lui Gauss
[tex]1 + 2 + 3 + ... + x = \\ \\ \huge= \frac{x \times (x + 1)}{2} [/tex]
unde x este ultimul număr
1)
[tex]1 + 2 + 3 + ... + 10 + 11 = \\ = \frac{11 \times 12}{2} = 11 \times 6 = 66[/tex]
2)
[tex]1 + 2 + 3 + .. + 50 + 51 = \\ = \frac{51 \times 52}{2} = 51 \times 26 = 1326[/tex]
3)
[tex]1 + 2 + 3 + ... + 89 = \\ = \frac{89 \times 90}{2} = 89 \times 45 = 4005[/tex]
