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D( z )
z = 0
z = 0
z = 0
z in (-oo:0) U (0:+oo)
3/5 = 4/z // - 4/z
3/5-(4/z) = 0
3/5-4*z^-1 = 0
-4*z^-1 = -3/5 // : -4
z^-1 = 3/20
-1 < 0
1/(z^1) = 3/20 // * z^1
1 = 3/20*z^1 // : 3/20
20/3 = z^1
z = 20/3
z = 20/3
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