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D( z )
z = 0
z = 0
z = 0
z in (-oo:0) U (0:+oo)
14/z = 9-z // - 9-z
z+14/z-9 = 0
1*z^1+14*z^-1-9*z^0 = 0
(1*z^2-9*z^1+14*z^0)/(z^1) = 0 // * z^2
z^1*(1*z^2-9*z^1+14*z^0) = 0
z^1
z^2-9*z+14 = 0
z^2-9*z+14 = 0
DELTA = (-9)^2-(1*4*14)
DELTA = 25
DELTA > 0
z = (25^(1/2)+9)/(1*2) or z = (9-25^(1/2))/(1*2)
z = 7 or z = 2
z in { 2, 7}
z in { 2, 7 }
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