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D( n )
n = 0
n = 0
n = 0
n in (-oo:0) U (0:+oo)
3/n = 7/49 // - 7/49
3/n-(7/49) = 0
3/n-1/7 = 0
3*n^-1 = 1/7 // : 3
n^-1 = 1/21
-1 < 0
1/(n^1) = 1/21 // * n^1
1 = 1/21*n^1 // : 1/21
21 = n^1
n = 21
n = 21
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