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D( r )
r = 0
r = 0
r = 0
r in (-oo:0) U (0:+oo)
1.5/6 = 8/r // - 8/r
1.5/6-(8/r) = 0
1.5/6-8*r^-1 = 0
-8*r^-1 = -0.25 // : -8
r^-1 = 0.03125
-1 < 0
1/(r^1) = 0.03125 // * r^1
1 = 0.03125*r^1 // : 0.03125
32 = r^1
r = 32
r = 32
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