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x in (-oo:+oo)
y = (1/2)*x^2-(6*x) // - (1/2)*x^2-(6*x)
6*x-((1/2)*x^2)+y = 0
(-1/2)*x^2+6*x+y = 0
6*x-1/2*x^2+y = 0
DELTA = 6^2-(-1/2*4*y)
DELTA = 2*y+36
2*y+36 = 0
2*y+36 = 0 // - 36
2*y = -36 // : 2
y = -36/2
y = -18
DELTA = 0 <=> t_1 = -18
x = -6/(-1/2*2) i y = -18
x = 6 i y = -18
( x = ((2*y+36)^(1/2)-6)/(-1/2*2) or x = (-(2*y+36)^(1/2)-6)/(-1/2*2) ) i y > -18
( x = 6-(2*y+36)^(1/2) or x = (2*y+36)^(1/2)+6 ) i y > -18
y+18 > 0
y+18 > 0 // - 18
y > -18
x in { 6, 6-(2*y+36)^(1/2), (2*y+36)^(1/2)+6 }
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