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D( x )
(200-x)/100 = 0
(200-x)/100 = 0
(200-x)/100 = 0
200-x = 0 // - 200
-x = -200 // * -1
x = 200
x in (-oo:200) U (200:+oo)
8.4 = (x/40)/((200-x)/100) // - (x/40)/((200-x)/100)
8.4-((x/40)/((200-x)/100)) = 0
(-5/2)*x*(200-x)^-1+8.4 = 0
(-5/2*x)/(200-x)+8.4 = 0
(-5/2*x)/(200-x)+(8.4*(200-x))/(200-x) = 0
8.4*(200-x)+(-5*x)/2 = 0
1680-10.9*x = 0
(1680-10.9*x)/(200-x) = 0
(1680-10.9*x)/(200-x) = 0 // * 200-x
1680-10.9*x = 0
1680-10.9*x = 0 // - 1680
-10.9*x = -1680 // : -10.9
x = -1680/(-10.9)
x = 154.12844037
x = 154.12844037
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