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D( x )
x = 0
x = 0
x = 0
x in (-oo:0) U (0:+oo)
4/x = 20/10 // - 20/10
4/x-(20/10) = 0
4/x-2 = 0
4*x^-1 = 2 // : 4
x^-1 = 1/2
-1 < 0
1/(x^1) = 1/2 // * x^1
1 = 1/2*x^1 // : 1/2
2 = x^1
x = 2
x = 2
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