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D( z )
z = 0
z = 0
z = 0
z in (-oo:0) U (0:+oo)
3/10 = 12/z // - 12/z
3/10-(12/z) = 0
3/10-12*z^-1 = 0
-12*z^-1 = -3/10 // : -12
z^-1 = 1/40
-1 < 0
1/(z^1) = 1/40 // * z^1
1 = 1/40*z^1 // : 1/40
40 = z^1
z = 40
z = 40
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