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D( b )
b = 0
b = 0
b = 0
b in (-oo:0) U (0:+oo)
8993/b = 391 // - 391
8993/b-391 = 0
8993*b^-1 = 391 // : 8993
b^-1 = 1/23
-1 < 0
1/(b^1) = 1/23 // * b^1
1 = 1/23*b^1 // : 1/23
23 = b^1
b = 23
b = 23
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