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D( r )
r = 0
r = 0
r = 0
r in (-oo:0) U (0:+oo)
2/3 = r/r-9 // - r/r-9
2/3-(r/r)+9 = 0
2/3-1+9 = 0
26/3 = 0
r belongs to the empty set
r belongs to the empty set
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