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D( n )
n = 0
n = 0
n = 0
n in (-oo:0) U (0:+oo)
80/20 = 4/n // - 4/n
80/20-(4/n) = 0
80/20-4*n^-1 = 0
-4*n^-1 = -4 // : -4
n^-1 = 1
-1 < 0
1/(n^1) = 1 // * n^1
1 = 1*n^1 // : 1
1 = n^1
n = 1
n = 1
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