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D( x )
3*x^2 = 0
3*x^2 = 0
3*x^2 = 0
3*x^2 = 0 // : 3
x^2 = 0
x = 0
x in (-oo:0) U (0:+oo)
(2*x^6)/(3*x^2) = 0
2/3*x^4 = 0 // : 2/3
x^4 = 0
x = 0
x in { 0}
x belongs to the empty set
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