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D( x )
2*x-1 = 0
2*x-1 = 0
2*x-1 = 0
2*x-1 = 0 // + 1
2*x = 1 // : 2
x = 1/2
x in (-oo:1/2) U (1/2:+oo)
(X^3-(6*x^2)-(21*x)-18)/(2*x-1) = 0
(X^3-6*x^2-21*x-18)/(2*x-1) = 0
X^3-6*x^2-21*x-18 = 0
DELTA = (-21)^2-(-6*4*(X^3-18))
DELTA = 24*(X^3-18)+441
24*(X^3-18)+441 = 0
24*(X^3-18)+441 = 0
24*X^3+9 = 0
24*X^3+9 = 0
24*X^3 = -9 // : 24
X^3 = -3/8
X^3 = -3/8 // ^ 1/3
X = -(3/8)^(1/3)
DELTA = 0 <=> t_2 = -(3/8)^(1/3)
x = 21/(-6*2) i X = -(3/8)^(1/3)
x = -7/4 i X = -(3/8)^(1/3)
( x = ((24*(X^3-18)+441)^(1/2)+21)/(-6*2) or x = (21-(24*(X^3-18)+441)^(1/2))/(-6*2) ) i X > -(3/8)^(1/3)
( x = ((24*(X^3-18)+441)^(1/2)+21)/(-12) or x = (21-(24*(X^3-18)+441)^(1/2))/(-12) ) i X > -(3/8)^(1/3)
X+(3/8)^(1/3) > 0
X+(3/8)^(1/3) > 0 // - (3/8)^(1/3)
X > -(3/8)^(1/3)
x in { -7/4, ((24*(X^3-18)+441)^(1/2)+21)/(-12), (21-(24*(X^3-18)+441)^(1/2))/(-12) }
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