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