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D( n )
n = 0
n = 0
n = 0
n in (-oo:0) U (0:+oo)
30/6 = 15/n // - 15/n
30/6-(15/n) = 0
30/6-15*n^-1 = 0
-15*n^-1 = -5 // : -15
n^-1 = 1/3
-1 < 0
1/(n^1) = 1/3 // * n^1
1 = 1/3*n^1 // : 1/3
3 = n^1
n = 3
n = 3
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