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calculati :1+2+3+....+80

Răspuns :

nx(n+1):2
80x81:2=
6480:2=3240
3240
[tex]\displaystyle 1+2+3+...+80= \frac{80(80+1)}{2} = \frac{80 \times 81}{2} = \frac{6480}{2} =3240[/tex]