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D( n )
n = 0
n = 0
n = 0
n in (-oo:0) U (0:+oo)
15/n = 1/40 // - 1/40
15/n-(1/40) = 0
15/n-1/40 = 0
15*n^-1 = 1/40 // : 15
n^-1 = 1/600
-1 < 0
1/(n^1) = 1/600 // * n^1
1 = 1/600*n^1 // : 1/600
600 = n^1
n = 600
n = 600
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