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D( r )
(r-6)^4 = 0
(r-6)^4 = 0
(r-6)^4 = 0
r-6 = 0 // + 6
r = 6
r in (-oo:6) U (6:+oo)
-54/((r-6)^4) = 0
r belongs to the empty set
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