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D( n )
n = 0
n = 0
n = 0
n in (-oo:0) U (0:+oo)
-12/2 = -7/n // + -7/n
-(-7/n)-12/2 = 0
7*n^-1-12/2 = 0
7*n^-1 = 6 // : 7
n^-1 = 6/7
-1 < 0
1/(n^1) = 6/7 // * n^1
1 = 6/7*n^1 // : 6/7
7/6 = n^1
n = 7/6
n = 7/6
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