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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 = (7*x)/(100-x) // - (7*x)/(100-x)
y-((7*x)/(100-x)) = 0
y-7*x*(100-x)^-1 = 0
(-7*x)/(100-x)+y = 0
(-7*x)/(100-x)+(y*(100-x))/(100-x) = 0
y*(100-x)-7*x = 0
100*y-7*x-(x*y) = 0
100*y-7*x-(x*y) = 0
x*(-y)+100*y = 0 // - 100*y
x*(-y) = -(100*y) // : -y
x = (-(100*y))/(-y)
x = 100
x-100 = 0
(x-100)/(100-x) = 0
(x-100)/(100-x) = 0 // * 100-x
x-100 = 0
x-100 = 0 // + 100
x = 100
x in { 100}
x belongs to the empty set
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