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D( r )
r = 0
r = 0
r = 0
r in (-oo:0) U (0:+oo)
6/r = 3/4 // - 3/4
6/r-(3/4) = 0
6/r-3/4 = 0
6*r^-1 = 3/4 // : 6
r^-1 = 1/8
-1 < 0
1/(r^1) = 1/8 // * r^1
1 = 1/8*r^1 // : 1/8
8 = r^1
r = 8
r = 8
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