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D( r )
r = 0
r = 0
r = 0
r in (-oo:0) U (0:+oo)
6/13 = 6/r // - 6/r
6/13-(6/r) = 0
6/13-6*r^-1 = 0
-6*r^-1 = -6/13 // : -6
r^-1 = 1/13
-1 < 0
1/(r^1) = 1/13 // * r^1
1 = 1/13*r^1 // : 1/13
13 = r^1
r = 13
r = 13
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