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D( r )
r = 0
r = 0
r = 0
r in (-oo:0) U (0:+oo)
12/r = 6 // - 6
12/r-6 = 0
12*r^-1 = 6 // : 12
r^-1 = 1/2
-1 < 0
1/(r^1) = 1/2 // * r^1
1 = 1/2*r^1 // : 1/2
2 = r^1
r = 2
r = 2
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