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D( X )
X = 0
X = 0
X = 0
X in (-oo:0) U (0:+oo)
(1*200)/X = 0.25 // - 0.25
(1*200)/X-0.25 = 0
200/X-0.25 = 0
200*X^-1 = 0.25 // : 200
X^-1 = 0.00125
-1 < 0
1/(X^1) = 0.00125 // * X^1
1 = 0.00125*X^1 // : 0.00125
800 = X^1
X = 800
X = 800
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