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D( x )
x = 0
x = 0
x = 0
x in (-oo:0) U (0:+oo)
6.4/x = 4 // - 4
6.4/x-4 = 0
6.4*x^-1 = 4 // : 6.4
x^-1 = 0.625
-1 < 0
1/(x^1) = 0.625 // * x^1
1 = 0.625*x^1 // : 0.625
1.6 = x^1
x = 1.6
x = 1.6
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