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D( r )
r = 0
r = 0
r = 0
r in (-oo:0) U (0:+oo)
267 = (200*(r+1))/r // - (200*(r+1))/r
267-((200*(r+1))/r) = 0
267-200*r^-1*(r+1) = 0
267-200*r^-1*(r+1) = 0
67-200*r^-1 = 0
67-200*r^-1 = 0
-200*r^-1 = -67 // : -200
r^-1 = 67/200
-1 < 0
1/(r^1) = 67/200 // * r^1
1 = 67/200*r^1 // : 67/200
200/67 = r^1
r = 200/67
r = 200/67
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