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D( z )
z = 0
z = 0
z = 0
z in (-oo:0) U (0:+oo)
7/z-3 = 4 // - 4
7/z-4-3 = 0
7*z^-1 = 7 // : 7
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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