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D( r )
r = 0
r = 0
r = 0
r in (-oo:0) U (0:+oo)
11/12 = 12/r // - 12/r
11/12-(12/r) = 0
11/12-12*r^-1 = 0
-12*r^-1 = -11/12 // : -12
r^-1 = 11/144
-1 < 0
1/(r^1) = 11/144 // * r^1
1 = 11/144*r^1 // : 11/144
144/11 = r^1
r = 144/11
r = 144/11
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