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D( z )
z = 0
z = 0
z = 0
z in (-oo:0) U (0:+oo)
1/z = 2/9 // - 2/9
1/z-(2/9) = 0
1/z-2/9 = 0
1*z^-1 = 2/9 // : 1
z^-1 = 2/9
-1 < 0
1/(z^1) = 2/9 // * z^1
1 = 2/9*z^1 // : 2/9
9/2 = z^1
z = 9/2
z = 9/2
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