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D( x )
x^4 = 0
x^4 = 0
x^4 = 0
1*x^4 = 0 // : 1
x^4 = 0
x = 0
x in (-oo:0) U (0:+oo)
((x^5*y^6)/(x^4))*y = 0
x*y^7 = 0 // : y^7
x = 0
x in { 0}
x belongs to the empty set
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