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D( r )
r = 0
r^2 = 0
r = 0
r = 0
r^2 = 0
r^2 = 0
1*r^2 = 0 // : 1
r^2 = 0
r = 0
r in (-oo:0) U (0:+oo)
(9*r)/(r^2)+r/r-64-8 = 0
9*r^-1 = 71 // : 9
r^-1 = 71/9
-1 < 0
1/(r^1) = 71/9 // * r^1
1 = 71/9*r^1 // : 71/9
9/71 = r^1
r = 9/71
r = 9/71
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