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D( a )
a = 0
a = 0
a = 0
a in (-oo:0) U (0:+oo)
7/5 = 21/a // - 21/a
7/5-(21/a) = 0
7/5-21*a^-1 = 0
-21*a^-1 = -7/5 // : -21
a^-1 = 1/15
-1 < 0
1/(a^1) = 1/15 // * a^1
1 = 1/15*a^1 // : 1/15
15 = a^1
a = 15
a = 15
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