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D( a )
a = 0
a = 0
a = 0
a in (-oo:0) U (0:+oo)
20/3 = 1/a // - 1/a
20/3-(1/a) = 0
20/3-a^-1 = 0
-1*a^-1 = -20/3 // : -1
a^-1 = 20/3
-1 < 0
1/(a^1) = 20/3 // * a^1
1 = 20/3*a^1 // : 20/3
3/20 = a^1
a = 3/20
a = 3/20
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