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D( r )
r = 0
r = 0
r = 0
r in (-oo:0) U (0:+oo)
10/r-5 = 8/11 // - 8/11
10/r-(8/11)-5 = 0
10/r-8/11-5 = 0
10*r^-1 = 63/11 // : 10
r^-1 = 63/110
-1 < 0
1/(r^1) = 63/110 // * r^1
1 = 63/110*r^1 // : 63/110
110/63 = r^1
r = 110/63
r = 110/63
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