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D( x )
x = 0
x = 0
x = 0
x in (-oo:0) U (0:+oo)
3 = 400/x // - 400/x
3-(400/x) = 0
3-400*x^-1 = 0
-400*x^-1 = -3 // : -400
x^-1 = 3/400
-1 < 0
1/(x^1) = 3/400 // * x^1
1 = 3/400*x^1 // : 3/400
400/3 = x^1
x = 400/3
x = 400/3
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