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D( x )
0.1-x = 0
0.1-x = 0
0.1-x = 0
0.1-x = 0 // - 0.1
-x = -0.1 // * -1
x = 0.1
x in (-oo:0.1) U (0.1:+oo)
60 = 3.9/(0.1-x) // - 3.9/(0.1-x)
60-(3.9/(0.1-x)) = 0
60-3.9*(0.1-x)^-1 = 0
60-3.9/(0.1-x) = 0
(60*(0.1-x))/(0.1-x)-3.9/(0.1-x) = 0
60*(0.1-x)-3.9 = 0
2.1-60*x = 0
(2.1-60*x)/(0.1-x) = 0
(2.1-60*x)/(0.1-x) = 0 // * 0.1-x
2.1-60*x = 0
2.1-60*x = 0 // - 2.1
-60*x = -2.1 // : -60
x = -2.1/(-60)
x = 0.035
x = 0.035
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