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x in (-oo:+oo)
(x*(x-1))/2 = 66 // - 66
(x*(x-1))/2-66 = 0
(x*(x-1))/2+(-66*2)/2 = 0
x*(x-1)-66*2 = 0
x^2-x-132 = 0
x^2-x-132 = 0
x^2-x-132 = 0
DELTA = (-1)^2-(-132*1*4)
DELTA = 529
DELTA > 0
x = (529^(1/2)+1)/(1*2) or x = (1-529^(1/2))/(1*2)
x = 12 or x = -11
(x+11)*(x-12) = 0
((x+11)*(x-12))/2 = 0
((x+11)*(x-12))/2 = 0 // * 2
(x+11)*(x-12) = 0
( x+11 )
x+11 = 0 // - 11
x = -11
( x-12 )
x-12 = 0 // + 12
x = 12
x in { -11, 12 }
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