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D( x )
x = 0
x = 0
x = 0
x in (-oo:0) U (0:+oo)
2/3 = -4/x // + -4/x
2/3-(-4/x) = 0
4*x^-1+2/3 = 0
4*x^-1 = -2/3 // : 4
x^-1 = -1/6
-1 < 0
1/(x^1) = -1/6 // * x^1
1 = -1/6*x^1 // : -1/6
-6 = x^1
x = -6
x = -6
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