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D( x )
7*x^21*y^21 = 0
7*x^21*y^21 = 0
7*x^21*y^21 = 0
7*x^21*y^21 = 0 // : 7*y^21
x^21 = 0
x = 0
x in (-oo:0) U (0:+oo)
(8*x^21*y^21)/(7*x^21*y^21) = 0
8/7 = 0
x belongs to the empty set
x belongs to the empty set
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