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D( X )
X = 0
X = 0
X = 0
X in (-oo:0) U (0:+oo)
1/3 = 9/X // - 9/X
1/3-(9/X) = 0
1/3-9*X^-1 = 0
-9*X^-1 = -1/3 // : -9
X^-1 = 1/27
-1 < 0
1/(X^1) = 1/27 // * X^1
1 = 1/27*X^1 // : 1/27
27 = X^1
X = 27
X = 27
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