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D( n )
n = 0
n = 0
n = 0
n in (-oo:0) U (0:+oo)
8/12 = 40/n // - 40/n
8/12-(40/n) = 0
8/12-40*n^-1 = 0
-40*n^-1 = -2/3 // : -40
n^-1 = 1/60
-1 < 0
1/(n^1) = 1/60 // * n^1
1 = 1/60*n^1 // : 1/60
60 = n^1
n = 60
n = 60
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