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D( z )
z^3 = 0
z^2 = 0
z^3 = 0
z^3 = 0
1*z^3 = 0 // : 1
z^3 = 0
z = 0
z^2 = 0
z^2 = 0
1*z^2 = 0 // : 1
z^2 = 0
z = 0
z in (-oo:0) U (0:+oo)
3/(z^2)-(7/(z^3)) = 0
3/(z^2)-7*z^-3 = 0
3*z^-2-7*z^-3 = 0
(3*z^1-7*z^0)/(z^3) = 0 // * z^6
z^3*(3*z^1-7*z^0) = 0
z^3
3*z-7 = 0
3*z^1 = 7 // : 3
z^1 = 7/3
z = 7/3
z in { 7/3}
z = 7/3
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