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D( a )
a = 0
a = 0
a = 0
a in (-oo:0) U (0:+oo)
28/a = 7 // - 7
28/a-7 = 0
28*a^-1 = 7 // : 28
a^-1 = 1/4
-1 < 0
1/(a^1) = 1/4 // * a^1
1 = 1/4*a^1 // : 1/4
4 = a^1
a = 4
a = 4
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