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D( r )
r-0.075 = 0
r-0.075 = 0
r-0.075 = 0
r-0.075 = 0 // + 0.075
r = 0.075
r in (-oo:0.075) U (0.075:+oo)
200 = 8.5/(r-0.075) // - 8.5/(r-0.075)
200-(8.5/(r-0.075)) = 0
200-8.5*(r-0.075)^-1 = 0
200-8.5/(r-0.075) = 0
(200*(r-0.075))/(r-0.075)-8.5/(r-0.075) = 0
200*(r-0.075)-8.5 = 0
200*r-23.5 = 0
(200*r-23.5)/(r-0.075) = 0
(200*r-23.5)/(r-0.075) = 0 // * r-0.075
200*r-23.5 = 0
200*r-23.5 = 0 // + 23.5
200*r = 23.5 // : 200
r = 23.5/200
r = 0.1175
r = 0.1175
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