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D( x )
x = 0
x = 0
x = 0
x in (-oo:0) U (0:+oo)
1-(9.89/x) = 0.25 // - 0.25
1-(9.89/x)-0.25 = 0
1-9.89*x^-1-0.25 = 0
-9.89*x^-1 = -0.75 // : -9.89
x^-1 = 0.07583418
-1 < 0
1/(x^1) = 0.07583418 // * x^1
1 = 0.07583418*x^1 // : 0.07583418
13.18666596 = x^1
x = 13.18666596
x = 13.18666596
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