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-2-4-6-...-80 va rog ajutați ma ​

Răspuns :

-2(1+2+3+...+40)

[tex]=-2(\frac{40(40+1)}{2} )\\\\=-2(\frac{40*41}{2})\\\\= -2*(20*41)\\\\=-2*820\\\\=-1640[/tex]