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D( r )
r = 0
r = 0
r = 0
r in (-oo:0) U (0:+oo)
35/42 = 24/r // - 24/r
35/42-(24/r) = 0
35/42-24*r^-1 = 0
-24*r^-1 = -5/6 // : -24
r^-1 = 5/144
-1 < 0
1/(r^1) = 5/144 // * r^1
1 = 5/144*r^1 // : 5/144
144/5 = r^1
r = 144/5
r = 144/5
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