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D( z )
z = 0
z = 0
z = 0
z in (-oo:0) U (0:+oo)
15/60 = 3/z // - 3/z
15/60-(3/z) = 0
15/60-3*z^-1 = 0
-3*z^-1 = -1/4 // : -3
z^-1 = 1/12
-1 < 0
1/(z^1) = 1/12 // * z^1
1 = 1/12*z^1 // : 1/12
12 = z^1
z = 12
z = 12
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