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D( n )
n = 0
n = 0
n = 0
n in (-oo:0) U (0:+oo)
21/n = 2/3 // - 2/3
21/n-(2/3) = 0
21/n-2/3 = 0
21*n^-1 = 2/3 // : 21
n^-1 = 2/63
-1 < 0
1/(n^1) = 2/63 // * n^1
1 = 2/63*n^1 // : 2/63
63/2 = n^1
n = 63/2
n = 63/2
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