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D( z )
z = 0
z = 0
z = 0
z in (-oo:0) U (0:+oo)
7/z = 5/12 // - 5/12
7/z-(5/12) = 0
7/z-5/12 = 0
7*z^-1 = 5/12 // : 7
z^-1 = 5/84
-1 < 0
1/(z^1) = 5/84 // * z^1
1 = 5/84*z^1 // : 5/84
84/5 = z^1
z = 84/5
z = 84/5
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