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D( x )
x^3 = 0
x^3 = 0
x^3 = 0
1*x^3 = 0 // : 1
x^3 = 0
x = 0
x in (-oo:0) U (0:+oo)
(x^7)/(x^3) = 0
1*x^4 = 0 // : 1
x^4 = 0
x = 0
x in { 0}
x belongs to the empty set
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