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D( r )
r = 0
r = 0
r = 0
r in (-oo:0) U (0:+oo)
2/r = -4 // + 4
2/r+4 = 0
2*r^-1 = -4 // : 2
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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