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D( x )
x = 0
x = 0
x = 0
x in (-oo:0) U (0:+oo)
3.5/x = 9 // - 9
3.5/x-9 = 0
3.5*x^-1 = 9 // : 3.5
x^-1 = 2.57142857
-1 < 0
1/(x^1) = 2.57142857 // * x^1
1 = 2.57142857*x^1 // : 2.57142857
0.38888889 = x^1
x = 0.38888889
x = 0.38888889
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