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D( d )
d = 0
d-3 = 0
d = 0
d = 0
d-3 = 0
d-3 = 0
d-3 = 0 // + 3
d = 3
d in (-oo:0) U (0:3) U (3:+oo)
(1/d-(1/3))/(d-3) = 0
(1/d-1/3)/(d-3) = 0
(d^-1-1/3)/(d-3) = 0
1*d^-1 = 1/3 // : 1
d^-1 = 1/3
-1 < 0
1/(d^1) = 1/3 // * d^1
1 = 1/3*d^1 // : 1/3
3 = d^1
d = 3
d in { 3}
d belongs to the empty set
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