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D( r )
r = 0
r = 0
r = 0
r in (-oo:0) U (0:+oo)
306/r = 34 // - 34
306/r-34 = 0
306*r^-1 = 34 // : 306
r^-1 = 1/9
-1 < 0
1/(r^1) = 1/9 // * r^1
1 = 1/9*r^1 // : 1/9
9 = r^1
r = 9
r = 9
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