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D( x )
x = 0
x = 0
x = 0
x in (-oo:0) U (0:+oo)
y = 8000/x // - 8000/x
y-(8000/x) = 0
y-8000*x^-1 = 0
-8000*x^-1 = -y // : -8000
x^-1 = y/8000
-1 < 0
1/(x^1) = y/8000 // * x^1
1 = x^1*(y/8000) // : y/8000
8000*y^-1 = x^1
x = 8000*y^-1
x = 8000*y^-1
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