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D( x )
(x-4)/(x^3) = 0
x^3 = 0
(x-4)/(x^3) = 0
(x-4)/(x^3) = 0
x-4 = 0 // + 4
x = 4
x^3 = 0
x^3 = 0
1*x^3 = 0 // : 1
x^3 = 0
x = 0
x in (-oo:0) U (0:4) U (4:+oo)
(((x-4)/(x^3))^-1)/18 = 0
(x^3)/(18*(x-4)) = 0
1*x^3 = 0 // : 1
x^3 = 0
x = 0
x in { 0}
x belongs to the empty set
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