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D( x )
x = 0
x = 0
x = 0
x in (-oo:0) U (0:+oo)
9/x+3 = 0
9*x^-1 = -3 // : 9
x^-1 = -1/3
-1 < 0
1/(x^1) = -1/3 // * x^1
1 = -1/3*x^1 // : -1/3
-3 = x^1
x = -3
x = -3
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