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D( x )
x^2-9 = 0
x^2-9 = 0
x^2-9 = 0
1*x^2 = 9 // : 1
x^2 = 9
x^2 = 9 // ^ 1/2
abs(x) = 3
x = 3 or x = -3
x in (-oo:-3) U (-3:3) U (3:+oo)
(3*x-9)/(x^2-9) = 0
3*x-9 = 0 // + 9
3*x = 9 // : 3
x = 9/3
x = 3
x in { 3}
x belongs to the empty set
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