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D( x )
2*5*x^4*x^4*y^3*y^4 = 0
2*5*x^4*x^4*y^3*y^4 = 0
2*5*x^4*x^4*y^3*y^4 = 0
10*x^8*y^7 = 0 // : 10*y^7
x^8 = 0
x = 0
x in (-oo:0) U (0:+oo)
(20*x^12*y^11)/(2*5*x^4*x^4*y^3*y^4) = 0
2*x^4*y^4 = 0 // : 2*y^4
x^4 = 0
x = 0
x in { 0}
x belongs to the empty set
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