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D( x )
x = 0
x = 0
x = 0
x in (-oo:0) U (0:+oo)
4/x = 12/28 // - 12/28
4/x-(12/28) = 0
4/x-3/7 = 0
4*x^-1 = 3/7 // : 4
x^-1 = 3/28
-1 < 0
1/(x^1) = 3/28 // * x^1
1 = 3/28*x^1 // : 3/28
28/3 = x^1
x = 28/3
x = 28/3
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