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D( z )
z+6 = 0
z = 0
z^2+6*z = 0
z+6 = 0
z+6 = 0
z+6 = 0 // - 6
z = -6
z = 0
z = 0
z^2+6*z = 0
z^2+6*z = 0
z^2+6*z = 0
DELTA = 6^2-(0*1*4)
DELTA = 36
DELTA > 0
z = (36^(1/2)-6)/(1*2) or z = (-36^(1/2)-6)/(1*2)
z = 0 or z = -6
z in (-oo:-6) U (-6:0) U (0:+oo)
1/(z+6)-(4/z) = (2*z+6)/(z^2+6*z) // - (2*z+6)/(z^2+6*z)
1/(z+6)-((2*z+6)/(z^2+6*z))-(4/z) = 0
1/(z+6)-((2*z+6)/(z^2+6*z))-4*z^-1 = 0
1/(z+6)+(-1*(2*z+6))/(z^2+6*z)-4/z = 0
z^2+6*z = 0
z^2+6*z = 0
z*(z+6) = 0
z+6 = 0 // - 6
z = -6
z*(z+6) = 0
1/(z+6)+(-1*(2*z+6))/(z*(z+6))-4/z = 0
(1*z)/(z*(z+6))+(-1*(2*z+6))/(z*(z+6))+(-4*(z+6))/(z*(z+6)) = 0
1*z-1*(2*z+6)-4*(z+6) = 0
-z-4*z-24-6 = 0
-5*z-30 = 0
(-5*z-30)/(z*(z+6)) = 0
(-5*z-30)/(z*(z+6)) = 0 // * z*(z+6)
-5*z-30 = 0
-5*z-30 = 0 // + 30
-5*z = 30 // : -5
z = 30/(-5)
z = -6
z in { -6}
z belongs to the empty set
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