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D( r )
r = 0
r^-4 = 0
r = 0
r = 0
r^-4 = 0
r^-4 = 0
1*r^-4 = 0 // : 1
r^-4 = 0
r należy do O
r in (-oo:0) U (0:+oo)
1/(r^-4) = 0
1*r^4 = 0 // : 1
r^4 = 0
r = 0
r in { 0}
r belongs to the empty set
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