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D( n )
n = 0
n = 0
n = 0
n in (-oo:0) U (0:+oo)
2500/3 = 4000/n // - 4000/n
2500/3-(4000/n) = 0
2500/3-4000*n^-1 = 0
-4000*n^-1 = -2500/3 // : -4000
n^-1 = 5/24
-1 < 0
1/(n^1) = 5/24 // * n^1
1 = 5/24*n^1 // : 5/24
24/5 = n^1
n = 24/5
n = 24/5
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