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D( x )
4*x-20 = 0
4*x-20 = 0
4*x-20 = 0
4*x-20 = 0 // + 20
4*x = 20 // : 4
x = 20/4
x = 5
x in (-oo:5) U (5:+oo)
(5-x)/(4*x-20) = 0
5-x = 0 // - 5
-x = -5 // * -1
x = 5
x in { 5}
x belongs to the empty set
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