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D( x )
5*x-15 = 0
9*x = 0
5*x-15 = 0
5*x-15 = 0
5*x-15 = 0 // + 15
5*x = 15 // : 5
x = 15/5
x = 3
9*x = 0
9*x = 0
9*x = 0 // : 9
x = 0
x in (-oo:0) U (0:3) U (3:+oo)
(((2*x-6)/(9*x))/(5*x-15))/3 = 0
(2*x-6)/(3*9*x*(5*x-15)) = 0
2*x-6 = 0 // + 6
2*x = 6 // : 2
x = 6/2
x = 3
x in { 3}
x belongs to the empty set
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