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D( r )
r = 0
r = 0
r = 0
r in (-oo:0) U (0:+oo)
13/r = 65/15 // - 65/15
13/r-(65/15) = 0
13/r-13/3 = 0
13*r^-1 = 13/3 // : 13
r^-1 = 1/3
-1 < 0
1/(r^1) = 1/3 // * r^1
1 = 1/3*r^1 // : 1/3
3 = r^1
r = 3
r = 3
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