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D( r )
r = 0
r = 0
r = 0
r in (-oo:0) U (0:+oo)
45/r = 9 // - 9
45/r-9 = 0
45*r^-1 = 9 // : 45
r^-1 = 1/5
-1 < 0
1/(r^1) = 1/5 // * r^1
1 = 1/5*r^1 // : 1/5
5 = r^1
r = 5
r = 5
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