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D( z )
z = 0
z = 0
z = 0
z in (-oo:0) U (0:+oo)
(9/7)*z+3/z = 1 // - 1
(9/7)*z+3/z-1 = 0
9/7*z^1+3*z^-1-1*z^0 = 0
(9/7*z^2-1*z^1+3*z^0)/(z^1) = 0 // * z^2
z^1*(9/7*z^2-1*z^1+3*z^0) = 0
z^1
(9/7)*z^2-z+3 = 0
(9/7)*z^2-z+3 = 0
DELTA = (-1)^2-(3*4*(9/7))
DELTA = -101/7
DELTA < 0
z in { }
z belongs to the empty set
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