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D( x )
0.42-x = 0
0.42-x = 0
0.42-x = 0
0.42-x = 0 // - 0.42
-x = -0.42 // * -1
x = 0.42
x in (-oo:0.42) U (0.42:+oo)
7 = (2*x)/(0.42-x) // - (2*x)/(0.42-x)
7-((2*x)/(0.42-x)) = 0
7-2*x*(0.42-x)^-1 = 0
(-2*x)/(0.42-x)+7 = 0
(-2*x)/(0.42-x)+(7*(0.42-x))/(0.42-x) = 0
7*(0.42-x)-2*x = 0
2.94-9*x = 0
(2.94-9*x)/(0.42-x) = 0
(2.94-9*x)/(0.42-x) = 0 // * 0.42-x
2.94-9*x = 0
2.94-9*x = 0 // - 2.94
-9*x = -2.94 // : -9
x = -2.94/(-9)
x = 0.32666667
x = 0.32666667
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