( -1 ) + ( -3 ) + ( -5 ) + ….. + ( -x ) = -2500

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1. `( -1 ) + ( -3 ) + ( -5 ) + ….. + ( -x ) = -2500`

   `⇔ 1 + 3  +5 + ….. + x  = 2500`

   `[(x-1)/2+1].(x+1):2=2500`

   `[(x-1)×(x+1)]/2+x+1=5000`

   `⇔(x^2+1+2x)/2=5000`

   `⇔(x+1)^2=10000=(±100)^2`

   `⇔x+1=100 hoặc x+1=-100`

   `⇔x=99hoặc x=-101`

    

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