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D( x )
4*x^5*y^-4 = 0
4*x^5*y^-4 = 0
4*x^5*y^-4 = 0
4*x^5*y^-4 = 0 // : 4*y^-4
x^5 = 0
x = 0
x in (-oo:0) U (0:+oo)
(4*x^5*y^-4)^-3*(((-8*x^6*y^3)^5)/3) = 0
-512/3*x^15*y^27 = 0 // : -512/3*y^27
x^15 = 0
x = 0
x in { 0}
x belongs to the empty set
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