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x in (-oo:+oo)
(x*(x+1))/2 = 100 // - 100
(x*(x+1))/2-100 = 0
(x*(x+1))/2+(-100*2)/2 = 0
x*(x+1)-100*2 = 0
x^2+x-200 = 0
x^2+x-200 = 0
x^2+x-200 = 0
DELTA = 1^2-(-200*1*4)
DELTA = 801
DELTA > 0
x = (801^(1/2)-1)/(1*2) or x = (-801^(1/2)-1)/(1*2)
x = (3*89^(1/2)-1)/2 or x = (-3*89^(1/2)-1)/2
(x-((-3*89^(1/2)-1)/2))*(x-((3*89^(1/2)-1)/2)) = 0
((x-((-3*89^(1/2)-1)/2))*(x-((3*89^(1/2)-1)/2)))/2 = 0
((x-((-3*89^(1/2)-1)/2))*(x-((3*89^(1/2)-1)/2)))/2 = 0 // * 2
(x-((-3*89^(1/2)-1)/2))*(x-((3*89^(1/2)-1)/2)) = 0
( x-((-3*89^(1/2)-1)/2) )
x-((-3*89^(1/2)-1)/2) = 0 // + (-3*89^(1/2)-1)/2
x = (-3*89^(1/2)-1)/2
( x-((3*89^(1/2)-1)/2) )
x-((3*89^(1/2)-1)/2) = 0 // + (3*89^(1/2)-1)/2
x = (3*89^(1/2)-1)/2
x in { (-3*89^(1/2)-1)/2, (3*89^(1/2)-1)/2 }
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