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D( x )
x-9 = 0
x-9 = 0
x-9 = 0
x-9 = 0 // + 9
x = 9
x in (-oo:9) U (9:+oo)
(9-x)/(x-9) = 0
9-x = 0 // - 9
-x = -9 // * -1
x = 9
x in { 9}
x belongs to the empty set
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