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D( x )
x = 0
x = 0
x = 0
x in (-oo:0) U (0:+oo)
2/3 = 85/x // - 85/x
2/3-(85/x) = 0
2/3-85*x^-1 = 0
-85*x^-1 = -2/3 // : -85
x^-1 = 2/255
-1 < 0
1/(x^1) = 2/255 // * x^1
1 = 2/255*x^1 // : 2/255
255/2 = x^1
x = 255/2
x = 255/2
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