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D( x )
x = 0
400/x <= 0
x = 0
x = 0
400/x <= 0
400/x <= 0
400*x^-1 <= 0
400*x^-1 <= 0 // : 400
x^-1 <= 0/400
x^-1 <= 0
1/(x^1) <= 0
x <> 0
1/(x^1) <= 0 // * x^2
(x^2)/(x^1) <= 0
x^1 <= 0
x <= 0
x in (-oo:0)
x in (0:+oo)
ln(400/x) = 20 // - 20
ln(400/x)-20 = 0
ln(400/x)-20 = 0 // + 20
ln(400/x) = 20
ln(400/x) = ln(e^20)
400/x = e^20
400/x-e^20 = 0
400*x^-1 = e^20 // : 400
x^-1 = (e^20)/400
-1 < 0
1/(x^1) = (e^20)/400 // * x^1
1 = ((e^20)/400)*x^1 // : (e^20)/400
400*e^-20 = x^1
x = 0
x in { 0}
x belongs to the empty set
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