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D( X )
12-(X/5000) = 0
12-(X/5000) = 0
12-(X/5000) = 0
12-1/5000*X = 0 // - 12
-1/5000*X = -12 // : -1/5000
X = -12/(-1/5000)
X = 60000
X in (-oo:60000) U (60000:+oo)
5000 = 22000/(12-(X/5000)) // - 22000/(12-(X/5000))
5000-(22000/(12-(X/5000))) = 0
5000-22000*(12-(X/5000))^-1 = 0
5000-22000/(12-1/5000*X) = 0
(5000*(12-1/5000*X))/(12-1/5000*X)-22000/(12-1/5000*X) = 0
5000*(12-1/5000*X)-22000 = 0
38000-X = 0
(38000-X)/(12-1/5000*X) = 0
(38000-X)/(12-1/5000*X) = 0 // * 12-1/5000*X
38000-X = 0
38000-X = 0 // - 38000
-X = -38000 // * -1
X = 38000
X = 38000
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