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D( r )
r = 0
r = 0
r = 0
r in (-oo:0) U (0:+oo)
1/5 = 50/r // - 50/r
1/5-(50/r) = 0
1/5-50*r^-1 = 0
-50*r^-1 = -1/5 // : -50
r^-1 = 1/250
-1 < 0
1/(r^1) = 1/250 // * r^1
1 = 1/250*r^1 // : 1/250
250 = r^1
r = 250
r = 250
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