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D( n )
n = 0
n = 0
n = 0
n in (-oo:0) U (0:+oo)
-105/15 = 21/n // - 21/n
-(21/n)-105/15 = 0
-21*n^-1-105/15 = 0
-21*n^-1 = 7 // : -21
n^-1 = -1/3
-1 < 0
1/(n^1) = -1/3 // * n^1
1 = -1/3*n^1 // : -1/3
-3 = n^1
n = -3
n = -3
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