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D( z )
z = 0
z = 0
z = 0
z in (-oo:0) U (0:+oo)
5/z-10 = -5 // + 5
5/z-10+5 = 0
5*z^-1 = 5 // : 5
z^-1 = 1
-1 < 0
1/(z^1) = 1 // * z^1
1 = 1*z^1 // : 1
1 = z^1
z = 1
z = 1
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