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D( x )
3*x-9 = 0
3*x-9 = 0
3*x-9 = 0
3*x-9 = 0 // + 9
3*x = 9 // : 3
x = 9/3
x = 3
x in (-oo:3) U (3:+oo)
1-(12/(3*x-9)) = 0
1-12*(3*x-9)^-1 = 0
1-12/(3*x-9) = 0
(1*(3*x-9))/(3*x-9)-12/(3*x-9) = 0
1*(3*x-9)-12 = 0
3*x-21 = 0
(3*x-21)/(3*x-9) = 0
(3*x-21)/(3*x-9) = 0 // * 3*x-9
3*x-21 = 0
3*x-21 = 0 // + 21
3*x = 21 // : 3
x = 21/3
x = 7
x = 7
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