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D( N )
N = 0
N = 0
N = 0
N in (-oo:0) U (0:+oo)
5/20 = 4/N // - 4/N
5/20-(4/N) = 0
5/20-4*N^-1 = 0
-4*N^-1 = -1/4 // : -4
N^-1 = 1/16
-1 < 0
1/(N^1) = 1/16 // * N^1
1 = 1/16*N^1 // : 1/16
16 = N^1
N = 16
N = 16
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