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D( r )
r = 0
r = 0
r = 0
r in (-oo:0) U (0:+oo)
3/r = 1/8 // - 1/8
3/r-(1/8) = 0
3/r-1/8 = 0
3*r^-1 = 1/8 // : 3
r^-1 = 1/24
-1 < 0
1/(r^1) = 1/24 // * r^1
1 = 1/24*r^1 // : 1/24
24 = r^1
r = 24
r = 24
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