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D( x )
x = 0
x = 0
x = 0
x in (-oo:0) U (0:+oo)
13 = 85/x // - 85/x
13-(85/x) = 0
13-85*x^-1 = 0
-85*x^-1 = -13 // : -85
x^-1 = 13/85
-1 < 0
1/(x^1) = 13/85 // * x^1
1 = 13/85*x^1 // : 13/85
85/13 = x^1
x = 85/13
x = 85/13
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