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D( r )
r = 0
r = 0
r = 0
r in (-oo:0) U (0:+oo)
5/6 = 4/r // - 4/r
5/6-(4/r) = 0
5/6-4*r^-1 = 0
-4*r^-1 = -5/6 // : -4
r^-1 = 5/24
-1 < 0
1/(r^1) = 5/24 // * r^1
1 = 5/24*r^1 // : 5/24
24/5 = r^1
r = 24/5
r = 24/5
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