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D( r )
r = 0
r = 0
r = 0
r in (-oo:0) U (0:+oo)
1/r = 1/9+1/16 // - 1/9+1/16
1/r-(1/9)-(1/16) = 0
1/r-1/9-1/16 = 0
1*r^-1 = 25/144 // : 1
r^-1 = 25/144
-1 < 0
1/(r^1) = 25/144 // * r^1
1 = 25/144*r^1 // : 25/144
144/25 = r^1
r = 144/25
r = 144/25
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