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D( z )
(3*z-3)^2 = 0
3*z-3 = 0
(3*z-3)^2 = 0
(3*z-3)^2 = 0
3*z-3 = 0 // + 3
3*z = 3 // : 3
z = 3/3
z = 1
3*z-3 = 0
3*z-3 = 0
3*z-3 = 0 // + 3
3*z = 3 // : 3
z = 3/3
z = 1
z in (-oo:1) U (1:+oo)
12/(3*z-3) = -20/((3*z-3)^2) // + -20/((3*z-3)^2)
12/(3*z-3)-(-20/((3*z-3)^2)) = 0
12/(3*z-3)+20*(3*z-3)^-2 = 0
12/(3*z-3)+20/((3*z-3)^2) = 0
(12*(3*z-3))/((3*z-3)^2)+20/((3*z-3)^2) = 0
12*(3*z-3)+20 = 0
36*z-16 = 0
(36*z-16)/((3*z-3)^2) = 0
(36*z-16)/((3*z-3)^2) = 0 // * (3*z-3)^2
36*z-16 = 0
36*z-16 = 0 // + 16
36*z = 16 // : 36
z = 16/36
z = 4/9
z = 4/9
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