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D( r )
r = 0
r = 0
r = 0
r in (-oo:0) U (0:+oo)
8/r-96 = 0
8*r^-1 = 96 // : 8
r^-1 = 12
-1 < 0
1/(r^1) = 12 // * r^1
1 = 12*r^1 // : 12
1/12 = r^1
r = 1/12
r = 1/12
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