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D( z )
z = 0
z = 0
z = 0
z in (-oo:0) U (0:+oo)
z-(7/z)-9 = 2/z-9 // - 2/z-9
z-(7/z)-(2/z)-9+9 = 0
z-7*z^-1-2*z^-1-9+9 = 0
1*z^1-9*z^-1 = 0
(1*z^2-9*z^0)/(z^1) = 0 // * z^2
z^1*(1*z^2-9*z^0) = 0
z^1
z^2-9 = 0
z^2-9 = 0
DELTA = 0^2-(-9*1*4)
DELTA = 36
DELTA > 0
z = (36^(1/2)+0)/(1*2) or z = (0-36^(1/2))/(1*2)
z = 3 or z = -3
z in { -3, 3}
z in { -3, 3 }
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