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D( r )
r = 0
r = 0
r = 0
r in (-oo:0) U (0:+oo)
10 = 5/r // - 5/r
10-(5/r) = 0
10-5*r^-1 = 0
-5*r^-1 = -10 // : -5
r^-1 = 2
-1 < 0
1/(r^1) = 2 // * r^1
1 = 2*r^1 // : 2
1/2 = r^1
r = 1/2
r = 1/2
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