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D( X )
X+6400 = 0
X+6400 = 0
X+6400 = 0
X+6400 = 0 // - 6400
X = -6400
X in (-oo:-6400) U (-6400:+oo)
X/(X+6400) = 5/1000 // - 5/1000
X/(X+6400)-(5/1000) = 0
X/(X+6400)-1/200 = 0
(200*X)/(200*(X+6400))+(-1*(X+6400))/(200*(X+6400)) = 0
200*X-1*(X+6400) = 0
199*X-6400 = 0
(199*X-6400)/(200*(X+6400)) = 0
(199*X-6400)/(200*(X+6400)) = 0 // * 200*(X+6400)
199*X-6400 = 0
199*X-6400 = 0 // + 6400
199*X = 6400 // : 199
X = 6400/199
X = 6400/199
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