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