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D( a )
a = 0
a = 0
a = 0
a in (-oo:0) U (0:+oo)
18/a-3+7 = (6*a)/a-3 // - (6*a)/a-3
18/a-((6*a)/a)-3+3+7 = 0
18/a-6-3+3+7 = 0
18*a^-1 = -1 // : 18
a^-1 = -1/18
-1 < 0
1/(a^1) = -1/18 // * a^1
1 = -1/18*a^1 // : -1/18
-18 = a^1
a = -18
a = -18
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