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D( r )
r = 0
r = 0
r = 0
r in (-oo:0) U (0:+oo)
1/r = 1/2+1/3 // - 1/2+1/3
1/r-(1/2)-(1/3) = 0
1/r-1/2-1/3 = 0
1*r^-1 = 5/6 // : 1
r^-1 = 5/6
-1 < 0
1/(r^1) = 5/6 // * r^1
1 = 5/6*r^1 // : 5/6
6/5 = r^1
r = 6/5
r = 6/5
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