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D( r )
2*r = 0
2*r = 0
2*r = 0
2*r = 0 // : 2
r = 0
r in (-oo:0) U (0:+oo)
((3*r)^2)/(2*r) = 0
9/2*r = 0 // : 9/2
r = 0
r in { 0}
r belongs to the empty set
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