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D( x )
x = 0
x = 0
x = 0
x in (-oo:0) U (0:+oo)
0.25/15 = 3.5/x // - 3.5/x
0.25/15-(3.5/x) = 0
0.25/15-3.5*x^-1 = 0
-3.5*x^-1 = -0.01666667 // : -3.5
x^-1 = 0.00476191
-1 < 0
1/(x^1) = 0.00476191 // * x^1
1 = 0.00476191*x^1 // : 0.00476191
209.999769 = x^1
x = 209.999769
x = 209.999769
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