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D( r )
r = 0
r = 0
r = 0
r in (-oo:0) U (0:+oo)
900 = 60/r // - 60/r
900-(60/r) = 0
900-60*r^-1 = 0
-60*r^-1 = -900 // : -60
r^-1 = 15
-1 < 0
1/(r^1) = 15 // * r^1
1 = 15*r^1 // : 15
1/15 = r^1
r = 1/15
r = 1/15
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