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D( x )
x = 0
x = 0
x = 0
x in (-oo:0) U (0:+oo)
y = 40/x // - 40/x
y-(40/x) = 0
y-40*x^-1 = 0
-40*x^-1 = -y // : -40
x^-1 = y/40
-1 < 0
1/(x^1) = y/40 // * x^1
1 = x^1*(y/40) // : y/40
40*y^-1 = x^1
x = 40*y^-1
x = 40*y^-1
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