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D( z )
z = 0
z = 0
z = 0
z in (-oo:0) U (0:+oo)
2-(3/z) = 12 // - 12
2-(3/z)-12 = 0
2-3*z^-1-12 = 0
-3*z^-1 = 10 // : -3
z^-1 = -10/3
-1 < 0
1/(z^1) = -10/3 // * z^1
1 = -10/3*z^1 // : -10/3
-3/10 = z^1
z = -3/10
z = -3/10
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