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D( r )
r = 0
r = 0
r = 0
r in (-oo:0) U (0:+oo)
5/3 = 2/r // - 2/r
5/3-(2/r) = 0
5/3-2*r^-1 = 0
-2*r^-1 = -5/3 // : -2
r^-1 = 5/6
-1 < 0
1/(r^1) = 5/6 // * r^1
1 = 5/6*r^1 // : 5/6
6/5 = r^1
r = 6/5
r = 6/5
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