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D( x )
15-(3*x) = 0
15-(3*x) = 0
15-(3*x) = 0
15-3*x = 0
15-3*x = 0 // - 15
-3*x = -15 // : -3
x = -15/(-3)
x = 5
x in (-oo:5) U (5:+oo)
2/(15-(3*x)) = 0
2/(15-3*x) = 0
x belongs to the empty set
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