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x in (-oo:+oo)
(2*x*(x+1))/2 = 12 // - 12
(2*x*(x+1))/2-12 = 0
(2*x*(x+1))/2+(-12*2)/2 = 0
2*x*(x+1)-12*2 = 0
2*x^2+2*x-24 = 0
2*x^2+2*x-24 = 0
2*(x^2+x-12) = 0
x^2+x-12 = 0
DELTA = 1^2-(-12*1*4)
DELTA = 49
DELTA > 0
x = (49^(1/2)-1)/(1*2) or x = (-49^(1/2)-1)/(1*2)
x = 3 or x = -4
2*(x+4)*(x-3) = 0
(2*(x+4)*(x-3))/2 = 0
(2*(x+4)*(x-3))/2 = 0 // * 2
2*(x+4)*(x-3) = 0
( x+4 )
x+4 = 0 // - 4
x = -4
( x-3 )
x-3 = 0 // + 3
x = 3
x in { -4, 3 }
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