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D( x )
100-x = 0
100-x = 0
100-x = 0
100-x = 0 // - 100
-x = -100 // * -1
x = 100
x in (-oo:100) U (100:+oo)
200/9 = (50*x)/(100-x) // - (50*x)/(100-x)
200/9-((50*x)/(100-x)) = 0
200/9-50*x*(100-x)^-1 = 0
(-50*x)/(100-x)+200/9 = 0
(-50*9*x)/(9*(100-x))+(200*(100-x))/(9*(100-x)) = 0
200*(100-x)-50*9*x = 0
20000-650*x = 0
(20000-650*x)/(9*(100-x)) = 0
(20000-650*x)/(9*(100-x)) = 0 // * 9*(100-x)
20000-650*x = 0
20000-650*x = 0 // - 20000
-650*x = -20000 // : -650
x = -20000/(-650)
x = 400/13
x = 400/13
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