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D( r )
r = 0
r = 0
r = 0
r in (-oo:0) U (0:+oo)
3/9 = 27/r // - 27/r
3/9-(27/r) = 0
3/9-27*r^-1 = 0
-27*r^-1 = -1/3 // : -27
r^-1 = 1/81
-1 < 0
1/(r^1) = 1/81 // * r^1
1 = 1/81*r^1 // : 1/81
81 = r^1
r = 81
r = 81
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