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D( X )
X = 0
X = 0
X = 0
X in (-oo:0) U (0:+oo)
13/X = 126/63 // - 126/63
13/X-(126/63) = 0
13/X-2 = 0
13*X^-1 = 2 // : 13
X^-1 = 2/13
-1 < 0
1/(X^1) = 2/13 // * X^1
1 = 2/13*X^1 // : 2/13
13/2 = X^1
X = 13/2
X = 13/2
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