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D( x )
3*x-12 = 0
3*x-12 = 0
3*x-12 = 0
3*x-12 = 0 // + 12
3*x = 12 // : 3
x = 12/3
x = 4
x in (-oo:4) U (4:+oo)
(2*x)/(3*x-12) = 0
2*x = 0 // : 2
x = 0
x = 0
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