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D( n )
n = 0
n = 0
n = 0
n in (-oo:0) U (0:+oo)
10/25 = 30/n // - 30/n
10/25-(30/n) = 0
10/25-30*n^-1 = 0
-30*n^-1 = -2/5 // : -30
n^-1 = 1/75
-1 < 0
1/(n^1) = 1/75 // * n^1
1 = 1/75*n^1 // : 1/75
75 = n^1
n = 75
n = 75
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