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D( n )
n = 0
n = 0
n = 0
n in (-oo:0) U (0:+oo)
2/12 = 7/n // - 7/n
2/12-(7/n) = 0
2/12-7*n^-1 = 0
-7*n^-1 = -1/6 // : -7
n^-1 = 1/42
-1 < 0
1/(n^1) = 1/42 // * n^1
1 = 1/42*n^1 // : 1/42
42 = n^1
n = 42
n = 42
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