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D( a )
a = 0
a = 0
a = 0
a in (-oo:0) U (0:+oo)
1-(15/a) = 6 // - 6
1-(15/a)-6 = 0
1-15*a^-1-6 = 0
-15*a^-1 = 5 // : -15
a^-1 = -1/3
-1 < 0
1/(a^1) = -1/3 // * a^1
1 = -1/3*a^1 // : -1/3
-3 = a^1
a = -3
a = -3
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