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D( R )
R = 0
R = 0
R = 0
R in (-oo:0) U (0:+oo)
5/R = 8/13 // - 8/13
5/R-(8/13) = 0
5/R-8/13 = 0
5*R^-1 = 8/13 // : 5
R^-1 = 8/65
-1 < 0
1/(R^1) = 8/65 // * R^1
1 = 8/65*R^1 // : 8/65
65/8 = R^1
R = 65/8
R = 65/8
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