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D( r )
r = 0
r = 0
r = 0
r in (-oo:0) U (0:+oo)
5/r-9 = 8/r+5 // - 8/r+5
5/r-(8/r)-9-5 = 0
5/r-8*r^-1-9-5 = 0
-3*r^-1 = 14 // : -3
r^-1 = -14/3
-1 < 0
1/(r^1) = -14/3 // * r^1
1 = -14/3*r^1 // : -14/3
-3/14 = r^1
r = -3/14
r = -3/14
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