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D( r )
r = 0
r = 0
r = 0
r in (-oo:0) U (0:+oo)
12/r = 3 // - 3
12/r-3 = 0
12*r^-1 = 3 // : 12
r^-1 = 1/4
-1 < 0
1/(r^1) = 1/4 // * r^1
1 = 1/4*r^1 // : 1/4
4 = r^1
r = 4
r = 4
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