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D( x )
5*x^3 = 0
5*x^3 = 0
5*x^3 = 0
5*x^3 = 0 // : 5
x^3 = 0
x = 0
x in (-oo:0) U (0:+oo)
(20*x^5)/(5*x^3) = 0
4*x^2 = 0 // : 4
x^2 = 0
x = 0
x in { 0}
x belongs to the empty set
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