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D( x )
x = 0
x = 0
x = 0
x in (-oo:0) U (0:+oo)
(x^20*(y/4))/x = 0
1/4*x^19*y = 0 // : 1/4*y
x^19 = 0
x = 0
x in { 0}
x belongs to the empty set
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