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D( z )
z = 0
z = 0
z = 0
z in (-oo:0) U (0:+oo)
8/z-(1/z) = 21 // - 21
8/z-(1/z)-21 = 0
8/z-z^-1-21 = 0
7*z^-1 = 21 // : 7
z^-1 = 3
-1 < 0
1/(z^1) = 3 // * z^1
1 = 3*z^1 // : 3
1/3 = z^1
z = 1/3
z = 1/3
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