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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)
1-(12/(3*z-3)) = -20/((3*z-3)^2) // + -20/((3*z-3)^2)
1-(12/(3*z-3))-(-20/((3*z-3)^2)) = 0
20*(3*z-3)^-2-12*(3*z-3)^-1+1 = 0
20/((3*z-3)^2)-12/(3*z-3)+1 = 0
(-12*(3*z-3))/((3*z-3)^2)+20/((3*z-3)^2)+(1*(3*z-3)^2)/((3*z-3)^2) = 0
1*(3*z-3)^2-12*(3*z-3)+20 = 0
9*z^2-36*z-18*z+9+56 = 0
9*z^2-54*z+65 = 0
9*z^2-54*z+65 = 0
9*z^2-54*z+65 = 0
DELTA = (-54)^2-(4*9*65)
DELTA = 576
DELTA > 0
z = (576^(1/2)+54)/(2*9) or z = (54-576^(1/2))/(2*9)
z = 13/3 or z = 5/3
(z-5/3)*(z-13/3) = 0
((z-5/3)*(z-13/3))/((3*z-3)^2) = 0
((z-5/3)*(z-13/3))/((3*z-3)^2) = 0 // * (3*z-3)^2
(z-5/3)*(z-13/3) = 0
( z-13/3 )
z-13/3 = 0 // + 13/3
z = 13/3
( z-5/3 )
z-5/3 = 0 // + 5/3
z = 5/3
z in { 13/3, 5/3 }
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