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D( x )
-4*x = 0
-4*x = 0
-4*x = 0
-4*x = 0 // : -4
x = 0
x in (-oo:0) U (0:+oo)
(12*x)/(-4*x) = 0
-3 = 0
x belongs to the empty set
x belongs to the empty set
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