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D( x )
x = 0
x = 0
x = 0
x in (-oo:0) U (0:+oo)
123.75/15 = 478.5/x // - 478.5/x
123.75/15-(478.5/x) = 0
123.75/15-478.5*x^-1 = 0
-478.5*x^-1 = -8.25 // : -478.5
x^-1 = 0.01724138
-1 < 0
1/(x^1) = 0.01724138 // * x^1
1 = 0.01724138*x^1 // : 0.01724138
57.99999768 = x^1
x = 57.99999768
x = 57.99999768
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