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D( z )
z = 0
z = 0
z = 0
z in (-oo:0) U (0:+oo)
8/z = 5/2 // - 5/2
8/z-(5/2) = 0
8/z-5/2 = 0
8*z^-1 = 5/2 // : 8
z^-1 = 5/16
-1 < 0
1/(z^1) = 5/16 // * z^1
1 = 5/16*z^1 // : 5/16
16/5 = z^1
z = 16/5
z = 16/5
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