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D( x )
x = 0
x = 0
x = 0
x in (-oo:0) U (0:+oo)
y = 1/x-4 // - 1/x-4
y-(1/x)+4 = 0
y-x^-1+4 = 0
-1*x^-1 = -(y+4) // : -1
x^-1 = y+4
-1 < 0
1/(x^1) = y+4 // * x^1
1 = x^1*(y+4) // : y+4
1/(y+4) = x^1
x = 1/(y+4)
x = 1/(y+4)
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