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D( z )
z = 0
z = 0
z = 0
z in (-oo:0) U (0:+oo)
9/z-6 = -z // + z
z+9/z-6 = 0
1*z^1+9*z^-1-6*z^0 = 0
(1*z^2-6*z^1+9*z^0)/(z^1) = 0 // * z^2
z^1*(1*z^2-6*z^1+9*z^0) = 0
z^1
z^2-6*z+9 = 0
z^2-6*z+9 = 0
DELTA = (-6)^2-(1*4*9)
DELTA = 0
z = 6/(1*2)
z = 3 or z = 3
z in { 3, 3}
z in { 3, 3 }
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