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D( R )
R = 0
R = 0
R = 0
R in (-oo:0) U (0:+oo)
80 = 1/R // - 1/R
80-(1/R) = 0
80-R^-1 = 0
-1*R^-1 = -80 // : -1
R^-1 = 80
-1 < 0
1/(R^1) = 80 // * R^1
1 = 80*R^1 // : 80
1/80 = R^1
R = 1/80
R = 1/80
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