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D( x )
3*x = 0
3*x = 0
3*x = 0
3*x = 0 // : 3
x = 0
x in (-oo:0) U (0:+oo)
y = 1/(3*x) // - 1/(3*x)
y-(1/(3*x)) = 0
(-1/3)*x^-1+y = 0
-1/3*x^-1 = -y // : -1/3
x^-1 = 3*y
-1 < 0
1/(x^1) = 3*y // * x^1
1 = 3*x^1*y // : 3*y
1/(3*y) = x^1
x = 1/(3*y)
x = 1/(3*y)
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