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1-4+7-10+13-16+.......+2017-2020
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Răspuns :

Rayzen

1-4+7-10+13-16+.......+2017-2020 =

= (3·1-2)-(3·2-2)+(3·3-2)-(3·4-2)+...+(3·673-2)-(3·674-2)

= 3·(1-2+3-4+...+673-674) -2+2-2+2-...-2+2

= 3·(-1-1-1-...-1)  {de 674/2 = 337 ori} + 0

= 3·(-337)

= -1011

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