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D( x )
x = 0
x = 0
x = 0
x in (-oo:0) U (0:+oo)
4/x = 6/15 // - 6/15
4/x-(6/15) = 0
4/x-2/5 = 0
4*x^-1 = 2/5 // : 4
x^-1 = 1/10
-1 < 0
1/(x^1) = 1/10 // * x^1
1 = 1/10*x^1 // : 1/10
10 = x^1
x = 10
x = 10
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