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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)
y = 9-(9*(100/(100-x))) // - 9-(9*(100/(100-x)))
9*(100/(100-x))+y-9 = 0
(9*100)/(100-x)+y-9 = 0
(9*100)/(100-x)+(y*(100-x))/(100-x)+(-9*(100-x))/(100-x) = 0
y*(100-x)-9*(100-x)+9*100 = 0
9*x-(x*y)+100*y-900+900 = 0
9*x-(x*y)+100*y = 0
9*x-(x*y)+100*y = 0
9*x+100*y = 0 // - 100*y
9*x = -(100*y) // : 9
x = (-(100*y))/9
x = (-100*y)/9
x+100/9*y = 0
(x+100/9*y)/(100-x) = 0
(x+100/9*y)/(100-x) = 0 // * 100-x
x+100/9*y = 0
x+100/9*y = 0 // - 100/9*y
x = -(100/9*y)
x = -100/9*y
x = -100/9*y
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