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