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D( n )
n = 0
n = 0
n = 0
n in (-oo:0) U (0:+oo)
12/4 = 21/n // - 21/n
12/4-(21/n) = 0
12/4-21*n^-1 = 0
-21*n^-1 = -3 // : -21
n^-1 = 1/7
-1 < 0
1/(n^1) = 1/7 // * n^1
1 = 1/7*n^1 // : 1/7
7 = n^1
n = 7
n = 7
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