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D( a )
a = 0
a = 0
a = 0
a in (-oo:0) U (0:+oo)
1/a-4 = 9 // - 9
1/a-9-4 = 0
1*a^-1 = 13 // : 1
a^-1 = 13
-1 < 0
1/(a^1) = 13 // * a^1
1 = 13*a^1 // : 13
1/13 = a^1
a = 1/13
a = 1/13
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