Tandreeaoanaask
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-1×(-1)¹+2×(-1)²-3×(-1)³+4×(-1)⁴-5×(-1)⁵+.......-2019 ×(-1)²⁰¹⁹​

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Andreutsa10ask

-1×(-1)¹+2×(-1)²-3×(-1)³+4×(-1)⁴-5×(-1)⁵+.......-2019 ×(-1)²⁰¹⁹=

-1×(-1)+2×1+-3×(-1)+......+-2019×(-1)=

1+2+3+4....+2019=

(1+2019)+(2+2018)+....+(1009+1011)+1010=

2020×1009 +1010=2039190

sau avem formula n(n+1)/2=2019×2020/2=2039190

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