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D( n )
n = 0
n = 0
n = 0
n in (-oo:0) U (0:+oo)
2/5 = 4/n // - 4/n
2/5-(4/n) = 0
2/5-4*n^-1 = 0
-4*n^-1 = -2/5 // : -4
n^-1 = 1/10
-1 < 0
1/(n^1) = 1/10 // * n^1
1 = 1/10*n^1 // : 1/10
10 = n^1
n = 10
n = 10
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